Marginal histograms ignore z/histfunc in density_heatmap/density_contour

[lucasjamar]

Related to #3521 (closed as stale in 2024, reopening as a fresh issue per maintainer request).

For 2D histograms and contour plots, marginal histograms (marginal_x/marginal_y="histogram") always show raw bin counts, regardless of the z value and histfunc passed to density_heatmap/density_contour. The main plot correctly aggregates z via histfunc, but the marginal subplots do not.

Example:

import plotly.express as px
df = px.data.tips()

fig = px.density_heatmap(
    df, x="size", y="tip", z="total_bill", histfunc="sum",
    marginal_x="histogram", marginal_y="histogram",
)
fig.show()

Expected: the marginal histograms show the sum of total_bill per bin, matching the main heatmap.
Actual: the marginal histograms show raw row counts per bin.

I have a fix ready and will open a PR referencing this issue.