create ggplot2 methods

[maxheld83]

The currently used pattern, where there's a plot_x() and a wrangle_x() function is a bit cumbersome and not super expressive.

Perhaps we want to go full ggplot2::autoplot() here at some point as per https://ggplot2.tidyverse.org/articles/ggplot2-in-packages.html.

This is on the backburner; ETL should be done first, because otherwise there's no point in defining methods if the objects change.

The preliminary pattern should work for now, because we're at least refactoring the plotting logic.