Refactoring plotting

[alexpghayes]

Where I'm at so far:

• I think having plot_pdf() and plot_cdf() are valuable even if there is some code repetition because the names are the most clear, and beginners in my experience have struggled with function arguments (i.e. type = "cdf" vs type = "pdf"), and we can take away that potential snag via specialized functions names at little additional maintenance cost.

• I don't think that having two distinct plotting systems is beginning friendly. I think it is important that there is one canonical way to do things in this package. I'm actually increasingly convinced aliasing plot() and autoplot() is going to cause beginner confusion.

• I don't particularly mind a ggplot2 hard dependency.

[zeileis]

• If you want to have separate plot_pdf() and plot_cdf() functions, I would implement these as simple convenience interfaces to the more flexible underlying plot() function. Essentially plot_pdf <- function(x, ...) plot(x, what = "pdf", ..) or something like that. This keeps the code DRY and facilitates maintenance while still providing shortcuts for beginners.
• Many teachers will not have covered ggplot2 when using this package for basic probability theory - and conversely many teachers will not go into base graphics but only cover ggplot2. So I would be inclusive to all of these and provide infrastructure for both plotting paradigms. This necessitates some explanation for beginners but I think that even most of them are quickly aware of that both systems exist and work rather differently.
• I'm not particularly worried about a ggplot2 dependency. I just think that so far it can be avoided relatively easily without disadvantages for those who want to use it. The benefit would be a leaner list of inherited dependencies for for future reverse dependencies that are likely to use mostly the distributional functionality and not necessarily the visualization infrastructure.