feat: Server side HoverTool for rasterized/datashaded plots with selector

[hoxbro]

Previously, we sent all hover data to the front end. This could lead to significant overhead on larger plots. With this PR, the data is pushed server side to the hover tool with a custom data model.

Screencast.2024-10-25.15.50.14.mp4

import datashader as ds
import numpy as np
import pandas as pd
import panel as pn

import holoviews as hv
from holoviews.operation.datashader import rasterize, datashade, dynspread

hv.extension("bokeh")

num = 10000
seed = np.random.default_rng(1)


dists = {
    cat: pd.DataFrame(
        {
            "x": seed.normal(x, s, num),
            "y": seed.normal(y, s, num),
            "s": s,
            "val": val,
            "cat": cat,
        }
    )
    for x, y, s, val, cat in [
        (2, 2, 0.03, 0, "d1"),
        (2, -2, 0.10, 1, "d2"),
        (-2, -2, 0.50, 2, "d3"),
        (-2, 2, 1.00, 3, "d4"),
        (0, 0, 3.00, 4, "d5"),
    ]
}

df = pd.concat(dists, ignore_index=True)
points = hv.Points(df)
agg, sel = ds.min("s"), ds.min("s")
plot = rasterize(points, aggregator=agg, selector=sel).opts(tools=["hover"])

Also adding support for datashaded plots:

plot = datashade(points, aggregator=agg, selector=sel).opts(tools=["hover"], hover_tooltips=["x", "y", "s"])

Screencast.2024-10-28.10.28.49.mp4

And dynspread:

plot = datashade(points, aggregator=agg, selector=sel).opts(tools=["hover"], hover_tooltips=["x", "y", "s", "cat"])
plot = dynspread(plot)

Screencast.2024-10-29.13.47.42.mp4

[codecov]

Codecov Report

Attention: Patch coverage is 84.82143% with 34 lines in your changes missing coverage. Please review.

Project coverage is 88.48%. Comparing base (3acfadb) to head (8fe1e26).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/tests/ui/bokeh/test_hover.py	51.28%	19 Missing ⚠️
holoviews/operation/datashader.py	89.04%	8 Missing ⚠️
holoviews/plotting/bokeh/raster.py	88.88%	6 Missing ⚠️
holoviews/element/raster.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6422      +/-   ##
==========================================
- Coverage   88.50%   88.48%   -0.03%     
==========================================
  Files         323      323              
  Lines       68631    68796     +165     
==========================================
+ Hits        60741    60872     +131     
- Misses       7890     7924      +34     

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