Make position_dodge2 preserve total width by default

[karawoo]

Closes #2290 by setting the default preserve argument in position_dodge2() to "total".

Before:

ggplot(mtcars, aes(x = factor(vs), y = mpg)) +
  facet_wrap( ~ factor(cyl)) +
  geom_boxplot()

After:

ggplot(mtcars, aes(x = factor(vs), y = mpg)) +
  facet_wrap( ~ factor(cyl)) +
  geom_boxplot()

[hadley]

Do you remember why we set it to single?

[karawoo]

It was because position_dodge2() is primarily used used for box plots, which can have variable widths but that only works when preserve = "single". So the reasoning was to make the default behavior of position_dodge2() compatible with geom_boxplot(varwidth = TRUE). But this causes the boxes to be too narrow in facetted plots.

I just pushed an update to make sure that preserve gets set back to "single" whenever varwidth = TRUE. We warn the user about the change when they explicitly asked for preserve = "total".

[hadley]

Thanks!

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