hover.tooltip convenience function for 'datetime' info

It would be nice to make it easy to create a “hover” tooltip template that could do some simple conversions/formatting on “standard” data point information: x, y, size, color, etc. Maybe also some convenience conversions for source attributes of data points.

My particular scenario has to do with providing a hover tooltip which displays a human-readable time stamp for the data point. The x-axis is configured as datetime, but when I display $x I get back a float (seconds since the epoch) which isn’t very readable. To get around this, I need to add a ColumnDataSource with a single field, like so:

source = ColumnDataSource(data=dict(
    time=df.map(lambda x: x.strftime('%d-%m-%Y'))
))

and then I tell my tooltip to display ("time", "@time"). This is workable, but it would be nicer if I could just do:

("time", "`stftime('%m-%d-%Y', $x)`"

or something similar to say “do a client-side conversion of $x into something more human-readable”

[bryevdv]

OK, scaling back to something slightly less ambitious. Will push a PR later today that provide for things like this:

p2.add_tools(HoverTool(tooltips=[("date", "@x{%F %T}")], formatters={"x": "datetime"}))

Which yields:

So formatters can optionally specify one of "datetime", "printf", or "numeral" for a CDS field name, which will use tz, SPrintf orNumbro respectively when displaying values from that field.

If no formatter is specified, it defaults to Numbro which preserves the current behavior

I think this leaves the door open for custom formatter models in the future, with something like

HoverTool(tooltips=[("date", "@x{custom}")], formatters={"x": my_custom_formatter})

but I don’t want to tackle that right now because I don’t want to tackle decision about what if anything to do to merge tick formatter and text formatters right now.

[bryevdv]

So this feature (to format unix timestamp as datetime in the hovertool) is requested since 2+ years, and not yet implemented in the latest bokeh release?

There is more work to do than is humanly possible accomplish by the number of people currently available to do it. New contributors are always welcome. That is literally the only thing that can cause any improvement in issue resolution times at this point