custom legend icons for geom_phylopic

[willgearty]

@LewisAJones requests that legends related to geom_phylopic should use the silhouettes instead of points

[willgearty]

I've been bashing my head against a wall trying to get this to work, but I'm not making any progress on this. I was originally trying to treat img as an aesthetic, so the legend could then have different silhouettes in it. However, it appears that that isn't possible with the way ggplot handles the rendering of legends.

However, I think I can make it such that the user can specify a silhouette to be used for the entire legend. This obviously wouldn't be that useful for plots with multiple different silhouettes, but could be very useful and aesthetically pleasing for plots with just a single unique silhouette.

[willgearty]

It might even be possible that, if only one unique silhouette is used in a plot, we just use that one automatically.

[LewisAJones]

Hmm... this should be relatively straightforward to implement. Ultimately, what we want to achieve is just changing the glyph in a legend.

I think the best way to do this is implementing a draw_key_phylopic. This tutorial shows an example of doing this with a smiley face. Maybe start there?

[willgearty]

Yes, it's straightforward to have a single custom image, but not straightforward to have the images be based on the images in the plot itself.

[willgearty]

After many attempts to make this automated, I've resorted to implementing a manual method for users to specify the silhouettes in the legend in the legend-keys branch. Have a look when you get a chance @LewisAJones.

[willgearty]

Some more information: the new method uses a package environment to store data between legend key generations. Unfortunately, this doesn't work for transferring information from the geom renderer to the legend renderer because the legend gets rendered first (ugh).