notes for future themes chapter PR

[djnavarro]

• Should explain the difference between + and %+replace. Seems to have gone missing in the current version? Possible example:

  base <- ggplot(mpg, aes(displ, hwy)) + geom_point()
  base + theme_dark() + theme(panel.background = element_rect(colour = "black"))
  base + theme_dark() %+replace% theme(panel.background = element_rect(colour = "black"))
  

• Needs an explicit section on theme inheritance? The intuitive model is to think it works like layers (adds new things) or scales (overrides previous) rather than the actual specificity-based model. The logic of this is stated in the current version but it is buried a little. Compare to https://ggplot2.tidyverse.org/reference/theme.html where inheritance is a top level heading. Among other things it may be worth discussing the element tree in the themes chapter. Something like this:

  element_tree <- get_element_tree()
  element_tree[["title"]]   # the title element does not inherit
  element_tree[["plot.title"]] # the plot.title element inherits from title
  

  It's not super exciting, but I think it will be useful in foreshadowing for the extensions chapter (i.e., it's hard to talk about modifying the element tree in an extension package if we haven't introduced the concept when describing the theming system)

• Discussion of ggsave() could be expanded? It's an oddly placed section: I almost wonder if there is value in having a short chapter discussing how ggplot objects interact with other systems? It could include saving plots, incorporating ggplot objects in R markdown, metadata (e.g., alt text), graphics devices, recent advances in fonts, etc?