marcskovmadsen/panel-holoviews

Panel + HoloViews Integration Patterns

• DO let Panel control the renderer theme
  • DON'T set hv.renderer('bokeh').theme = 'dark_minimal'

---

DynamicMap: Preserve Zoom/Pan Across Data Refreshes

When you set pane.object = new_plot, Bokeh resets all axes ranges. Wrap the plot function in hv.DynamicMap so Bokeh updates data in the existing figure rather than replacing it.

DON'T: Replace chart object directly

# BAD — zoom resets every refresh
self._chart_pane.object = df.hvplot.scatter(...)

DO: Use DynamicMap with a trigger parameter

class Monitor(pn.viewable.Viewer):
    _chart_trigger = param.Integer(default=0)

    def __init__(self, **params):
        super().__init__(**params)
        dmap = hv.DynamicMap(pn.bind(self._render_scatter, self.param._chart_trigger))
        self._chart_pane = pn.pane.HoloViews(dmap, sizing_mode="stretch_width")

    def _render_scatter(self, trigger):
        # Reads self.data directly; trigger is just a signal to re-invoke
        df = self.data
        if df is None or df.empty:
            return hv.Scatter([], kdims=['x'], vdims=['y']).opts(responsive=True, height=300)
        # Pass responsive and height directly to hvplot — see "Responsive Sizing" section
        return df.hvplot.scatter(x='x', y='y', responsive=True, height=300)

    def _on_data_changed(self, *events):
        # Increment trigger → DynamicMap re-invokes → Bokeh patches in place
        self._chart_trigger += 1

---

One Element Per DynamicMap

Returning an hv.Overlay from a DynamicMap causes two problems:

1. Type mismatch errors — if you sometimes return hv.Scatter and sometimes hv.Overlay, DynamicMap raises AssertionError: DynamicMap must only contain one type of object.
2. Lost hover tooltips — when scatter + HLines are combined inside hv.Overlay([...]), the scatter's hover tool configuration doesn't propagate.

DON'T: Return mixed types or Overlays from a single DynamicMap

# BAD — type mismatch when data is empty vs populated
def render(trigger):
    if no_data:
        return hv.Text(0, 0, "empty")  # Text type
    plot = df.hvplot.scatter(...)
    return plot * hv.HLine(avg)  # Overlay type → AssertionError

# BAD — hover tooltips lost
def render(trigger):
    scatter = df.hvplot.scatter(..., tools=['hover'])
    return hv.Overlay([scatter, hv.HLine(avg)])  # hover doesn't propagate

DO: Separate DynamicMap per element, combine with * at layout level

def __init__(self, **params):
    super().__init__(**params)
    scatter_dmap = hv.DynamicMap(pn.bind(self._render_scatter, self.param._trigger))
    avg_dmap = hv.DynamicMap(pn.bind(self._render_avg_line, self.param._trigger))
    min_dmap = hv.DynamicMap(pn.bind(self._render_min_line, self.param._trigger))
    max_dmap = hv.DynamicMap(pn.bind(self._render_max_line, self.param._trigger))
    overlay = scatter_dmap * avg_dmap * min_dmap * max_dmap
    self._chart_pane = pn.pane.HoloViews(overlay, sizing_mode="stretch_width")

Each callback returns exactly one element type, always:

def _render_scatter(self, trigger):
    completed = self._get_completed()
    if completed.empty:
        # Same type as the populated case — just no data
        return hv.Scatter([], kdims=['START_TIME'], vdims=['RUNTIME_SECONDS']).opts(
            bgcolor='#0d1015', height=180, responsive=True,
        )
    return completed.hvplot.scatter(x='START_TIME', y='RUNTIME_SECONDS')

def _render_avg_line(self, trigger):
    avg = self._stats().get('avg_runtime', 0)
    if avg > 0:
        return hv.HLine(avg).opts(color='orange', line_dash='dashed')
    # Invisible but valid — same type always
    return hv.HLine(0).opts(alpha=0)

Benefits:

• Each DynamicMap always returns the same HoloViews element type (no AssertionError)
• Scatter keeps its hover tools natively (tools are per-element, not per-overlay)
• Each layer updates independently
• Static layers (like a tile source) can be pulled out of DynamicMap entirely

Note: Element * DynamicMap or DynamicMap * DynamicMap yields a DynamicMap, not a static hv.Overlay. This is expected — Panel handles it the same way via pn.pane.HoloViews.

---

Responsive Sizing

HoloViews .opts(width=, height=, responsive=) and Panel's sizing_mode on pn.pane.HoloViews are two separate sizing systems. They conflict if misconfigured.

DO: Pass responsive=True and height directly to the hvplot call

# hvplot: pass responsive and height as arguments so hvplot does NOT set a default fixed width
plot = df.hvplot.scatter(x='time', y='value', responsive=True, height=300)
pane = pn.pane.HoloViews(plot, sizing_mode="stretch_width")

# Pure HoloViews: .opts() is fine because HoloViews doesn't inject a default width
plot = hv.Curve(df, 'time', 'value').opts(responsive=True, height=300)
pane = pn.pane.HoloViews(plot, sizing_mode="stretch_width")

DON'T: Use .opts(responsive=True) on an hvplot object

# BAD — hvplot internally sets width=700; .opts(responsive=True) doesn't remove it.
# Bokeh sees both fixed width and responsive=True → warning + broken layout.
# Inside a DynamicMap the plot shrinks on every refresh.
plot = df.hvplot.scatter(x='time', y='value').opts(responsive=True, height=300)

# BAD — same problem with explicit fixed width
plot = df.hvplot.scatter(...).opts(width=600, height=300)
pane = pn.pane.HoloViews(plot, sizing_mode="stretch_width")

Key rules:

• hvplot: always pass responsive=True + height=<int> as arguments to the hvplot call (e.g. df.hvplot.scatter(..., responsive=True, height=300)), and sizing_mode="stretch_width" on the pane. Do NOT use .opts() for these — hvplot sets a default width=700 internally and .opts() cannot remove it.
• Pure HoloViews: using .opts(responsive=True, height=<int>) is fine because HoloViews elements don't inject a default fixed width.
• For fixed size: set width=<int> + height=<int> in .opts(), omit sizing_mode on the pane
• DON'T mix fixed width in opts with sizing_mode on the pane — Panel overrides the width but emits a warning; the conflict makes behavior ambiguous
• Never set both width and responsive=True in .opts() — width wins and responsive is silently ignored

---

HoloViews Streams with Panel

HoloViews streams let you react to user interactions on plots (clicks, selections, zoom) inside a Panel app. Attach streams to a DynamicMap; Panel wires up the callbacks automatically.

Selection1D: React to selected points

import holoviews as hv
from holoviews import streams

points = hv.Points(df, kdims=['x', 'y']).opts(tools=['tap', 'box_select'], size=8)
selection = streams.Selection1D(source=points)

def show_selected(index):
    if not index:
        return hv.Table(df.iloc[:0], kdims=['x', 'y'])
    return hv.Table(df.iloc[index], kdims=['x', 'y'])

table_dmap = hv.DynamicMap(show_selected, streams=[selection])
pn.Row(points, table_dmap).servable()

RangeXY: React to zoom/pan range

curve = hv.Curve(df, 'time', 'value')
range_stream = streams.RangeXY(source=curve)

def show_stats(x_range, y_range):
    if x_range is None:
        return hv.Text(0, 0, "Zoom to select range")
    x0, x1 = x_range
    subset = df[(df['time'] >= x0) & (df['time'] <= x1)]
    return hv.Text(0, 0, f"Mean: {subset['value'].mean():.2f}")

stats_dmap = hv.DynamicMap(show_stats, streams=[range_stream])
pn.Column(curve, stats_dmap).servable()

Tap: React to click location

points = hv.Points(df, kdims=['x', 'y']).opts(tools=['tap'], size=8)
tap_stream = streams.Tap(source=points)

def on_tap(x, y):
    if x is None:
        return hv.Text(0, 0, "Click a point")
    return hv.Text(x, y, f"({x:.1f}, {y:.1f})").opts(text_font_size='12pt')

tap_dmap = hv.DynamicMap(on_tap, streams=[tap_stream])
(points * tap_dmap).servable()

Pipe / Buffer: Push streaming data

from holoviews.streams import Pipe, Buffer

# Pipe — replace data entirely each push
pipe = Pipe(data=[])
# framewise=True: recompute axis ranges on each push (without it, axes lock to first frame)
pipe_dmap = hv.DynamicMap(hv.Curve, streams=[pipe]).opts(framewise=True)

def update():
    pipe.send(new_dataframe)

# Buffer — append data, keep last N rows
buffer = Buffer(df.iloc[:0], length=500)
buffer_dmap = hv.DynamicMap(hv.Curve, streams=[buffer]).opts(framewise=True)

def update():
    buffer.send(new_rows_df)

Common stream pitfalls

• Forgot tools: Selection1D needs tools=['tap', 'box_select'] in .opts() — without them no events fire
• Unhandled None: Stream callbacks receive None/empty values on first render — always guard with if not index: or if x is None:
• Don't mix mechanisms: Use either streams OR param.depends/pn.bind for a given plot — not both
• Frozen axes with Pipe/Buffer: By default DynamicMap locks axis ranges to the first frame. Use .opts(framewise=True) so axes update when data ranges change

---

Linked Selections / Cross-Filtering

hv.link_selections provides automatic cross-filtering across plots. LLMs often try to build this manually with streams — use the built-in API instead.

Basic usage

import holoviews as hv
from holoviews.operation import histogram

ls = hv.link_selections.instance()

scatter = hv.Points(df, kdims=['x', 'y'])
# DO use hv.operation.histogram — preserves link to source data so
# link_selections can filter by all source dimensions (x AND y)
hist_x = histogram(scatter, dimension='x', num_bins=20)
hist_y = histogram(scatter, dimension='y', num_bins=20)

# Wrap each plot — selections in one propagate to all
layout = ls(scatter) + ls(hist_x) + ls(hist_y)
pn.pane.HoloViews(layout, sizing_mode="stretch_width").servable()

Categorical bars: Custom Operation for categorical aggregation

hv.operation.histogram only works for numeric dimensions. For categorical bar charts (e.g. counting trades per commodity, or computing mean P&L per region), you need a custom Operation subclass. Because operations wrap the source element, link_selections can "unwrap" them to access all original dimensions when resolving cross-filter expressions.

from holoviews.core import Operation

class categorical_agg(Operation):
    """Aggregate a categorical dimension, returning Bars.

    Preserves data lineage back to the source element so that
    link_selections can resolve all source dimensions during cross-filtering.
    """

    dimension = param.String(doc="Categorical dimension to group by")
    value_dimension = param.String(
        default=None,
        allow_None=True,
        doc="Numeric dimension to aggregate. None means count rows.",
    )
    function = param.Callable(
        default=np.size,
        doc="Aggregation function (np.sum, np.mean, np.std, np.min, np.max, ...)",
    )
    label = param.String(
        default=None,
        allow_None=True,
        doc="Label for the value axis. Auto-generated if None.",
    )

    def _process(self, element, key=None):
        cat_vals = element.dimension_values(self.p.dimension, expanded=True)
        unique_cats = np.unique(cat_vals)

        if self.p.value_dimension is None:
            # Pure count
            _, counts = np.unique(cat_vals, return_counts=True)
            agg_label = self.p.label or "Count"
            data = list(zip(unique_cats, counts))
        else:
            num_vals = element.dimension_values(self.p.value_dimension, expanded=True)
            results = []
            for cat in unique_cats:
                mask = cat_vals == cat
                results.append(self.p.function(num_vals[mask]))
            func_name = getattr(self.p.function, "__name__", "agg")
            agg_label = self.p.label or f"{func_name}({self.p.value_dimension})"
            data = list(zip(unique_cats, results))

        return hv.Bars(data, kdims=[self.p.dimension], vdims=[agg_label])

# Usage with link_selections:
ls = hv.link_selections.instance()
points = hv.Points(df, kdims=['price', 'volume'],
                   vdims=['commodity', 'region', 'trade_type', 'pnl'])

hist_price = histogram(points, dimension='price', num_bins=20)           # numeric → histogram
bars_commodity = categorical_agg(points, dimension='commodity')          # categorical count (default)
bars_avg_pnl = categorical_agg(points, dimension='commodity',           # categorical mean
                               value_dimension='pnl', function=np.mean)

layout = ls(points) + ls(hist_price) + ls(bars_commodity) + ls(bars_avg_pnl)

Why this works: HoloViews operations maintain a reference to their source element. When link_selections encounters a selection expression like (dim('price') >= 50), it recurses into the operation's source (points) where price exists, applies the filter, then re-runs the operation on the filtered data. Pre-aggregated hv.Bars lack this source reference, so the expression cannot resolve and raises CallbackError.

DON'T: Use pre-binned histograms, pre-aggregated data, or histogram on categorical dimensions

# BAD — pre-binned histogram loses data lineage; link_selections can't resolve
# the scatter's 'y' dimension on the histogram → CallbackError
hist = hv.Histogram(np.histogram(df['x'], bins=20), kdims='x')

# BAD — pre-aggregated bars lose the original x/y columns;
# selection expressions referencing those dimensions → CallbackError
bars = hv.Bars(df.groupby('cat').size().reset_index(name='n'), kdims='cat', vdims='n')

# BAD — histogram() only supports numeric dimensions;
# raises "Cannot create histogram from categorical data"
bars = histogram(points, dimension='commodity')

# BAD — hv.Bars(raw_df, kdims=['cat']) auto-detects ALL other columns as vdims,
# including datetime and string columns that cause DType errors during rendering
bars = hv.Bars(df, kdims=['commodity'])  # timestamp vdim → DTypePromotionError

Accessing the selection for filtering

ls = hv.link_selections.instance()
# ... build linked plots ...

# selection_expr is a HoloViews dim expression — apply it to a Dataset, not a raw DataFrame
ds = hv.Dataset(df, kdims=["x", "y"])
expr = ls.selection_expr
if expr is not None:
    mask = expr.apply(ds)        # returns a boolean numpy array
    filtered_df = df[mask]

Key rules:

• DO use .instance() to create the link_selections object — calling hv.link_selections(plot) directly returns a plot, not a reusable linker
• DO apply selection_expr to a hv.Dataset, not a pandas DataFrame — expr.apply(df) raises AttributeError
• DO use hv.operation.histogram(source_element, dimension='x') for histograms — this preserves the data lineage so link_selections can filter by all source dimensions. DON'T use hv.Histogram(np.histogram(...)) — pre-binned histograms lose the source data link, and link_selections raises CallbackError when the selection expression references dimensions not present on the histogram
• DO use a custom Operation subclass (like categorical_agg above) for categorical bar charts — this preserves data lineage the same way histogram does for numeric dimensions. Supports count (default), sum, mean, std, min, max, or any custom callable via the function parameter. DON'T use histogram() on categorical dimensions (it raises an error) and DON'T use pre-aggregated groupby().size() bars (they lose the source data link)
• DON'T add selection tools manually — link_selections adds box_select and lasso_select automatically
• Each linked plot must share the same dimension names for cross-filtering to work
• Dependencies: link_selections requires pyarrow at runtime for selection display. Lasso selection additionally requires shapely (or spatialpandas). Install both: pip install pyarrow shapely. Without pyarrow, selections silently fail with CallbackError; without shapely, lasso raises ImportError while box-select still works

---

Client-Side Interactions with jslink

For styling/visual controls that don't need Python computation, use jslink to wire Panel widgets directly to Bokeh properties. No server roundtrip, works in saved HTML files.

Simple property binding

# Float slider → glyph fill alpha (also works for size, line_width, fill_color, etc.)
widget = pn.widgets.FloatSlider(value=1, step=0.01)
plot = hv.Points((x, y)).opts(size=10)
widget.jslink(plot, value='glyph.fill_alpha')

# Text input → plot title
widget = pn.widgets.TextInput(value="Title")
plot = hv.Curve((x, y)).opts(title="Title")
widget.jslink(plot, value="plot.title.text")

# Text input → axis label
widget = pn.widgets.TextInput(value="X Label")
plot = hv.Curve((x, y)).opts(xlabel="X Label")
widget.jslink(plot, value="xaxis.axis_label")

# RadioButtonGroup → title alignment
widget = pn.widgets.RadioButtonGroup(options=["left", "center", "right"])
widget.jslink(plot, value="plot.title.align")

JavaScript code callbacks

For transforms that need a bit of JS logic, use the code parameter:

# Range slider → axis limits
widget = pn.widgets.RangeSlider(start=0, end=10)
plot = hv.Curve((x, y))
widget.jslink(plot, code={'value': """
    x_range.start = cb_obj.value[0];
    x_range.end = cb_obj.value[1];
"""})

Bokeh property targets available via jslink:

• glyph.* — fill_alpha, fill_color, size, line_width, line_color, etc.
• plot.title.* — text, text_font_size, align
• xaxis.* / yaxis.* — axis_label, etc.
• x_range.* / y_range.* — start, end
• color_mapper.* — low, high, palette

When to use jslink vs DynamicMap:

• jslink — pure visual/styling changes, axis limits, color tweaks. No Python needed.
• DynamicMap — data transformations, aggregations, anything that needs Python computation.

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pn.pane.HoloViews Configuration

| Parameter | Type | Description | |---|---|---| | linked_axes | bool (True) | Link axes ranges across plots in a layout | | widget_location | str ('right') | Location of groupby/DynamicMap widgets: 'left', 'right', 'top', 'bottom', etc. | | center | bool (False) | Center the plot in the pane | | theme | str/Theme (None) | Bokeh theme override — use instead of hv.renderer().theme | | backend | str ('bokeh') | HoloViews plotting backend | | sizing_mode | str (None) | Panel sizing: 'stretch_width', 'stretch_both', 'fixed', etc. |

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Summary Checklist

| Pattern | Do | Don't | |---|---|---| | Preserve zoom | hv.DynamicMap(pn.bind(fn, trigger)) | pane.object = new_plot | | Overlay composition | One DynamicMap per element, * at layout | hv.Overlay([...]) inside callback | | Empty chart state | hv.Scatter([], kdims=..., vdims=...) | hv.Overlay([]) or hv.Text(...) | | Responsive width (hvplot) | df.hvplot(..., responsive=True, height=N) + sizing_mode="stretch_width" on pane | .opts(responsive=True) after hvplot (hvplot's default width=700 persists, conflicts) | | Responsive width (HoloViews) | .opts(responsive=True, height=N) + sizing_mode="stretch_width" on pane | Fixed width in opts (conflicts with responsive pane, emits warning) | | React to selections | streams.Selection1D(source=plot) + DynamicMap | Manual click tracking with param.depends | | Cross-filtering | hv.link_selections.instance() | Building manual stream wiring | | Cross-filter categorical bars | categorical_agg Operation subclass on source element | Pre-aggregated groupby().size() bars or histogram() on categorical dims | | Streaming data | Pipe (replace) or Buffer (append) streams | Replacing pane.object in a loop | | Renderer theme | pn.pane.HoloViews(plot, theme=...) | hv.renderer('bokeh').theme = ... | | Client-side styling | widget.jslink(plot, value='glyph.fill_alpha') | DynamicMap for pure visual tweaks |