should we drop the vertical line in mcmc_areas() and mcmc_areas_ridges()?

[jgabry]

I'm starting to think that maybe we shouldn't be hardcoding the vertical line at zero at all. Users don't have the option to remove and it would be super easy for them to add a vertical afterwards if they want it. We could consider suggesting users add + vline_0() (or + geom_vline(xintercept=0) directly) after making the plot if they want the vertical line.

I'm not saying we should definitely change this, just thinking out loud. Of course if someone wants the plot without the line they do already have the option of getting the data from mcmc_*_data() and making it themselves.

[jgabry]

I suppose another option is to have an argument to toggle it.

[avehtari]

Makes sense either way