plotter: add Thumbnail method to HeatMap
Created by: kortschak
A heatmap needs a colour scale in the legend. This means HeatMap needs to satisfy plot.Thumbnailer. There may be additional requirements, but this is the first step.
Created by: kortschak
A heatmap needs a colour scale in the legend. This means HeatMap needs to satisfy plot.Thumbnailer. There may be additional requirements, but this is the first step.