Missing elements in theme definitions

[clauswilke]

All themes miss some elements in their definitions (i.e., elements are implicitly set to NULL rather than explicitly set to NULL). I think all themes provided by ggplot2 should explicitly define all elements. This will become more important if we want to allow extensible themes (#2540, #2784), because we'll have to distinguish between elements a theme knows about but wants to be NULL and elements a theme doesn't know about that need to be filled in from the theme defaults. Missing element definitions also have led to unexpected problems in the past, e.g. #2724.

Elements that I notice are missing:

• In all themes: some of panel.grid, panel.grid.major, panel.grid.minor, panel.grid.major.x, panel.grid.minor.x, panel.grid.major.y, panel.grid.minor.y
• In theme_void(): Too many to list, including parts of axis.*, legend.*, panel.*, strip.*, plot.background.

As an aside, we should recommend as a general practice to external theme developers that they always start with an existing ggplot2 theme and then just modify the parts they want to change, just like we do in most cases:

ggplot2/R/theme-defaults.r

Lines 247 to 267 in 6f5ffea

	# Starts with theme_grey and then modify some parts
	theme_grey(
	base_size = base_size,
	base_family = base_family,
	base_line_size = base_line_size,
	base_rect_size = base_rect_size
	) %+replace%
	theme(
	# white background and dark border
	panel.background = element_rect(fill = "white", colour = NA),
	panel.border = element_rect(fill = NA, colour = "grey20"),
	# make gridlines dark, same contrast with white as in theme_grey
	panel.grid = element_line(colour = "grey92"),
	panel.grid.minor = element_line(size = rel(0.5)),
	# contour strips to match panel contour
	strip.background = element_rect(fill = "grey85", colour = "grey20"),
	# match legend key to background
	legend.key = element_rect(fill = "white", colour = NA),
	complete = TRUE
	)

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