Add descenders to heights

[hadley]

And increase margins slightly. Fixes #1712

@baptiste, @thomasp85, @jiho – can you please take a look? I always forget the consequences of tweaking theme defaults.

library(ggplot2)
df <- data.frame(x = 1:9, y = 1:9)

ggplot(df, aes(x, y)) + 
  geom_point() + 
  labs(
    y = "yyy",
    title = "This is title yyyyyy",
    subtitle = "This is a subtitle yyyyyy"
  )

Before:

After:

The difference is subtle, but an improvement.

[jiho]

I like the change.

With descenders taken into account, I think the extra space may not be needed (I prefer more room for the data and less for the legends).
In particular, I would put the left margin of axis.title.y and the bottom margin of axis.title.x to 0. This way, when setting plot.margin to 0, the titles are flush with the border of the plot, like the panel background and the main title are. I usually want this for plots inserted within a text, so that the left of the y axis title aligns exactly with the left of the paragraphs and the x axis falls on the baseline if the height of the plot is correctly computed.
Regarding the 0.8 * half_line vs. half_line for the other margin I have no strong opinion.

This is the current with branch descenders

and this has the outer margins at 0

and this would be with outer margins at 0 and plot.margin at 0 (with titles with ascenders and descenders):

(open the images to better see the position with respect to the margin).

[hadley]

The top of the T gets cut off in the last plot, but I could certainly drop the outer padding to something nominal like 0.1.

[jiho]

I'm guessing the added comma is problematic here. It should create an error.

[jiho]

Indeed, the top of capitals or ascenders and the bottom of descenders are slightly cut off (although that depends on the rendering software; when I import the PDF into Inkscape and re-export it, the clipping of letters is not as pronounced). And indeed, I think the solution is to set plot.margin at 0.1 instead of 0, rather than add outer margins the to various outer plot elements (because you would need to add it to the title but also to the panel if there is no title etc.).

So basically, the changes look perfect to me now.

[jiho]

There was a way at some point to change the position of the axes (put the x axis on top, the y axis on the right, etc.). See https://groups.google.com/forum/?fromgroups=#!topic/ggplot2-dev/JkJU5CLBkQw

Is that not relevant anymore? If such tweaks exist then, ideally, there should be a way in the themes to specify "outer margin" instead of left/right/top/bottom margin. The easy alternative of course would be to make margins symmetrical, but I would be less pleasing visually as explained above.

[thomasp85]

This is not currently possible, but depending on the complexity it might be included in the facet rewrite (with @hadley's blessing of course)

with regards to margins, see my comment on #1727 - optimally this proposal should be implemented for all margin settings and would allow for symmetric margin setting

[hadley]

I think the theming system is already sufficiently complicated without having to additionally distinguish between inner/outer. The unfortunate reality is that if you have a theme where the labels are in a different place you will have to tweak margin settings in numerous locations. I don't think there is a good solution for this apart from implementing a full constraint-based layout system.

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