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

[hoxbro]

Previously, we sent all hover data to the front end, which 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.

Currently working on datashade(agg=by, sel=min) and datashade(agg=by, sel=min).

Example script

import itertools

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 datashade, dynspread as _dynspread, rasterize

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)
df["y"] += 100  # Offset to distinguish x from y
points = hv.Points(df)


def dynspread(plot, aggregator, selector):
    return _dynspread(datashade(plot, aggregator=aggregator, selector=selector))


ops = (rasterize, datashade, dynspread)
aggs = (ds.min("s"), ds.by("cat"))
sels = (None, ds.min("s"))
combinations = itertools.product(ops, aggs, sels)
plots = []
for op, agg, sel in combinations:
    title = f"{op.__name__}(agg={type(agg).__name__}, sel={type(sel).__name__ if sel else None})"
    try:
        plot = op(points, aggregator=agg, selector=sel).opts(
            tools=["hover"], title=title, width=400, height=400
        )
    except Exception as e:
        print(e)
        plot = hv.Curve([]).opts(title=title, width=400, height=400)
    plots.append(plot)

pn.panel(hv.Layout(plots).cols(4).opts(shared_axes=False)).servable()

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 36 lines in your changes missing coverage. Please review.

Project coverage is 88.48%. Comparing base (3acfadb) to head (725d46b).
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	88.88%	9 Missing ⚠️
holoviews/plotting/bokeh/raster.py	87.50%	7 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    68806     +175     
==========================================
+ Hits        60741    60880     +139     
- Misses       7890     7926      +36     

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