raster-aggregation in matplotlib extension

[raphaelquast]

Is your feature request related to a problem? Please describe.

I've already stated the problem on discourse (here), but since it does not seem to get a lot of
attention there I thought I'll bring this up in a more general way on github as well.

I'm the dev of EOmaps, which already provides an integration for datashader via the matplotlib-extension.

While working on some improvements on the link between EOmaps and datashader, I've noticed
that at the moment, (at least as I understand it) the dsshow function of the matplotlib-extension
only allows for bypixel aggregations, which limits the capabilities quite a bit.

To be more precise, I've been trying to get "raster"-like aggregations (using for example "mode" reduction) working but
I could not find a way on how to do that without monkey-patching parts of the matplotlib-extension (see below)

Describe the solution you'd like

A clear and concise description of what you want to happen.

at the moment, the aggregation in dsshow() is implemented like this:

canvas = Canvas(
    plot_width=plot_width,
    plot_height=plot_height,
    x_range=x_range,
    y_range=y_range,
)
binned = bypixel(self.df, canvas, self.glyph, self.aggregator)

however, as I understand it, a more flexible way would be to use something like

canvas = Canvas(
    plot_width=plot_width,
    plot_height=plot_height,
    x_range=x_range,
    y_range=y_range,
)

# <aggregation-type> = "raster", "point", "line" etc. 
# <reduction> = "mean", "median", "max" etc.
agg = canvas.<aggregation-type>(<Dataset>, agg=<reduction>)
binned = agg.compute()

This is just a scetch to clarify what I mean...

Describe alternatives you've considered

Well, I've managed to get raster-aggregation working in EOmaps by monkey-patching the aggregate() function of the matplotlib-extension, but I don't think that this is a proper (and sustainable) way to provide a better datashader-integration.

Additional context

here's what I intend to achieve (ideally without having to temper with the matpltolib-extension myself):