Add a scatter plot to compare two bands

[ivandorte]

It would be nice to add a scatter plot to investigate the relationships between different combinations of bands (e.g. red vs. nir). Then we could add a tap stream to pick a pixel from the map and see how it is collocated in this scatter plot.

[ivandorte]

WIP

The stream works, but it is very inefficient; next steps:

• Implement in Params;
• Plot the selected point on map;
• A better colormap;
• Draw a roi (or a polygon, freehand?) on the scatter plot and mark the matching pixels in the map;

[lalligagger]

Ok - no problem to push this out a bit if it'll be a bigger effort! Looks very cool though. Definitely like the idea of selectable ROI which will help the load. Could also think about using an agg function so it isn't every pixel but some binning.