Make plot title position configurable.

[clauswilke]

Closes #3252.

library(ggplot2)

p <- ggplot(mpg, aes(x = class)) +
  geom_bar() +
  coord_flip() +
  labs(
    title = "SUVs are the most popular",
    subtitle = "Number of cars of different classes in the mpg dataset") +
  theme(axis.title.y = element_blank())

p

p + theme(plot.title.position = "plot")

^{Created on 2019-08-19 by the reprex package (v0.3.0)}

[clauswilke]

@thomasp85 Any opinions on this approach? I'm happy to finish this up if there are no major objections.

[thomasp85]

This is more or less how I was planning to implement it. It will complicate patchwork but that is how it is... enough people have been hacking at ggplot2 to get this.

How does this work with the default tag position?

[clauswilke]

Tag appears to the top left of the plot, which seems reasonable to me. People may have to fiddle a bit with margins around the tag, but it's essentially fine as is.

library(ggplot2)

ggplot(mpg, aes(x = class)) +
  geom_bar() +
  coord_flip() +
  labs(
    title = "SUVs are the most popular",
    subtitle = "Number of cars of different classes in the mpg dataset",
    tag = "A") +
  theme(axis.title.y = element_blank(), plot.title.position = "plot")

^{Created on 2019-08-24 by the reprex package (v0.3.0)}

[thomasp85]

Yeah, I think this looks fine

[clauswilke]

I think this is ready for review now.

[thomasp85]

LGTM, but @hadley may have opinions

[hadley]

LGTM. This is definitely something we should highlight in the release blog post.

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