Display hovertool for the uppermost glyph in bokeh?

Question

I have a graph with several hundreds of glyphs on it, generated in bokeh. All the glyphs have a hovertool associated with them that shows a popup window with an image in it when moused over.

Some of the glyphs have 4-5 glyphs layered underneath them, and when someone mouses over them, all the hovertools are displayed.

Is it possible to only display the hovertool for the uppermost glyph?

Answer 1

As of Bokeh 1.3.4 there is no way to control the number of tooltips displayed without creating some sort of a custom extension. There is an open issue #9087 Provide max_tooltips property on HoverTool that is targeted for one of the next few releases.

But note: that work will just take a tooltip out of multiple to show. The draw order for a single glyph is not well-defined, it can change due to: spatial indexing, selections, "Level of Detail" mode being active, and other things. I don't know that there will ever be a reliable notion of "total z-order" such that eg the "top" circle could be the one picked (and always be the same index). A more sophisticated selection manager could sort or condition on other CDS column values, etc. though, so you could potentially explicitly provide the order you intend.