Zoom/pan events for charts

[jrieke]

Checklist

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Summary

We will soon add selections to charts (st.plotly_chart and st.altair_chart). At some point, we should also add a way to react to zoom/pan events on charts and return the bounding box of the displayed chart.

Why?

Let us know in the comments if you have specific use cases in mind!

How?

We should probably add a new event parameter (e.g. on_zoom_pan or on_relayout or on_move). Or we could think about integrating it with on_select in some way (which would make the implementation a lot easier but might be a bit less obvious from UX perspective).

Additional Context

Altair already supports this via selections events: using .interactive() in Altair adds a selection interval event (https://altair-viz.github.io/user_guide/interactions.html#scale-binding) and is a shortcut for alt.selection_interval(bind='scales'). You can use that to get the area the user is currently viewing.

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