plot_last: make plots the same size

[FelixTheStudent]

Plot_last creates two plots:

• rule plot, colours indicate TRUE/FALSE
• feature plot, colour indicates gene expression levels

It's notoriously difficult to make the two plots align vertically, because they are different in width. The rel_widths ratio varies depending on how many digits the maximum expression has (as this is displayed in feat's legend).

Solution I'd like to implement:

• make both plots a little wider, by extending the x-axis (increasing the maximal UMAP1 coordinate value)
• for feat, put the legend into this area as inset graph. Does that work?!
• for rule, put TRUE / FALSE together with the corresponding number of cells there, colored same as the points.

[FelixTheStudent]

The two plots now have the same width, and the CP10K legend is included as third plot. plot_last has the legend_rel_width argument to control the size of this legend.