Add phylopic_key_glyph

[willgearty]

This adds a custom key glyph function that can be used with geom_phylopic to specify silhouettes for the legend.

Resolves #57.

[LewisAJones]

This works really well @willgearty. I know it gave you some pain, so great job. A few things we might want to consider:

• Ensure this works with the updated aesthetics for geom_phylopic (use of colour and fill) and outline feature.
• Can we make this work with the size aesthetic as well?
• Do we want an option for creating legends for add_phylopic and add_phylopic_base?

[willgearty]

Note: I still need to figure out why one of the tests isn't making the correct snapshot.

[willgearty]

With the most recent commit, looks like this "just works" with the new fill/color split.

[LewisAJones]

Hi @willgearty, this is working well! Last thing from me is whether we expect color defined outside of the aesthetics to also work when multiple values are defined? I think we should be encouraging it to be defined in the aesthetics, but just in case, I thought I'd mention it.

library(ggplot2)
df <- data.frame(x = c(2, 4), y = c(10, 20),
                 name = c("Felis silvestris catus", "Odobenus rosmarus"))
# Works with outline colour
ggplot(df) +
  geom_phylopic(aes(x = x, y = y, name = name, fill = name), color = c("blue"), 
                size = 10,
                show.legend = TRUE,
                key_glyph = phylopic_key_glyph(name =
                                                 c("Felis silvestris catus",
                                                   "Odobenus rosmarus"))) +
  coord_cartesian(xlim = c(1,6), ylim = c(5, 30))
# Doesn't work with outline colour (defaults to black)
ggplot(df) +
  geom_phylopic(aes(x = x, y = y, name = name, fill = name), color = c("blue", "red"), 
                size = 10,
                show.legend = TRUE,
                key_glyph = phylopic_key_glyph(name =
                                                 c("Felis silvestris catus",
                                                   "Odobenus rosmarus"))) +
  coord_cartesian(xlim = c(1,6), ylim = c(5, 30))

[willgearty]

This is the expected behavior for a ggplot legend (which should only represent the aesthetics). For example:

ggplot(df) +
    geom_point(aes(x = x, y = y, fill = name), color = c("blue", "red"), size = 10, shape = 21) +
    coord_cartesian(xlim = c(1,6), ylim = c(5, 30))

[willgearty]

OK, your two other requests/comments have been moved to separate issues for future development. I'll merge this once the checks pass!