Strange rasterize colormapping in Large Data user guide

[jlstevens]

While discussing #4567 with @jbednar, we noticed the following strange output in the Large Data User Guide

This looks pretty strange as it looks like the reverse of the fire colormap where black is low and white is high as the raster holds positive counts with higher counts in the middle:

Initially I thought this might be related to the recent work in #4567 but then realized that it can't be as that PR hasn't been merged yet.

Presumably it used to look like the GIF shown for the next code cell (i.e normal fire with a black background)?

[jlstevens]

@philippjfr did say this was rather strange. As it is important to address, I'll assign it to the next milestone.

[jlstevens]

I've tracked this down and it looks like the fire colormap is currently inverted from what it should be:

whereas 'fire_r' looks like 'fire':

This can't always have been wrong as the GIF above that is part of the notebook shows fire correctly. I have no idea how this happened but we should try to track it down as it is concerning.

[jlstevens]

Adding hv.opts.defaults(hv.opts.Image(cmap='fire_r')) to the top of the Large Data user guide 'fixes' this issue but doesn't explain why fire is reversed from what it should be.