remove extra space from mcmc_areas

[avehtari]

I needed a mcmc_areas plot for a book, and noticed that all mcmc_areas plots have unnecessary extra space on top and bottom. This is not clear with just a few variables, but with 4 than more it's visible e.g. in bodyfat, candy, diabetes, mesquite, roaches, and winequality case studies at https://avehtari.github.io/modelselection/
It gets worse and worse with more variables.
For example this kind of call has empty space

mcmc_areas(as.matrix(fit), pars=vars(-'(Intercept)',-sigma))

and it can be fixed with

p0 <- mcmc_areas(as.matrix(fit0), pars=vars(-'(Intercept)',-sigma)
p0 <- p0 + scale_y_discrete(limits = rev(levels(p0$data$parameter)))
p0

This approach is a bit annoying, so I propose to make the plots tight by default.

This had slipped past previously as in notebooks there is plenty of space, but for a book I need tight layout and then I noticed that the extra space is actually appearing in many plots.

[jgabry]

@avehtari Thanks for bringing this up and sorry for the slow response. I finally made some time recently to dig back into bayesplot issues.

This was a bit tricky because if we don't add the expand argument to scale_y_discrete() then sometimes the tops of the areas get slightly cut off, but the settings we were using definitely created way too much space on top.

@tjmahr might have a better idea, but after playing around with a bunch of options and trying with different numbers of parameters, what ended up working best (I think) was to use an additive expansion that decreases with the number of parameters. I put this on a branch remove-extra-space-mcmc_areas. Here's a comparison with the current master branch:

For 2, 4, 10, and 20 parameters (simulated from standard normals) here is what the current master branch produces:

and here is what the version on the new branch remove-extra-space-mcmc_areas looks like for the same parameters:

What do you think?

[avehtari]

New version looks much better. It would be useful to add in the documentation a comment how to adjust the spacing manually, if even more tight spacing is desired.

[jgabry]

Thanks for checking. Good idea, I'll add an example in the Examples section of the doc.