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