Support iTOL-style "leaf sorting"

[fedarko]

This would sort clades in the rectangular and circular layouts by the number of descendant leaves (see the iTOL docs here). I've seen this strategy used in lots of tree figures in papers, and it looks pretty :), so I assume it would be nice if this was an option. (Would be easiest to implement on the command line, but be useful to have in the browser.)

Currently, Empress doesn't do this kind of sorting -- so the original order of clades in the tree's Newick file is followed.

update: I've implemented this in the leafsorting branch of my fork (https://github.com/fedarko/empress/tree/leafsorting). I imagine it would be easiest to revisit this once the current prototype branch is finalized and merged in.

[fedarko]

For reference, here are some screenshots showing how this looks. There are three options:

• no leaf sorting (how things are currently drawn in Empress)
• leaf sorting, with clades sorted in descending order by number of leaves (how iTOL's "normal mode" works by default)
• leaf sorting, with clades sorted in ascending order by number of leaves (opposite of the above, included here because I messed the sorting up the first time and thought it looked cool)

These are all on the moving pictures tree (which I admit may not be the best test case for this due to its handful of super long branches). I'm also including iTOL's visualizations of the tree for comparison (although that tree is uncolored, so this is less of a competition re: "omg which looks better????" and more of a "basically everyone uses iTOL because it is super useful, so let's see how closely our layout choices resemble theirs to save everyone's sanity").

Rectangular layout

No leaf sorting

With leaf sorting (descending order)

With leaf sorting (ascending order)

iTOL (had to set the Vert scaling factor to 0.05 to get the entire tree to display on the screen; I believe this is because Empress' rectangular layout uses a relatively small distance between tips' y-coordinates, whereas iTOL's default is much larger)

Circular layout

No leaf sorting

With leaf sorting (descending order)

With leaf sorting (ascending order)

iTOL (to be consistent with our circular layout as is, I changed the rotation to 0 degrees from default 210; also changed arc to 360 degrees from default 350)

Unrooted layout

(Substituting in the leaf-sorted postorder traversal for the postorder traversal in the unrooted layout doesn't seem to make a difference -- all of the Empress plots look basically the same, so I'm not posting an extra set of screenshots here.)