ppc ribbon with points for observations

[charlesm93]

Per the discussion on https://discourse.mc-stan.org/t/bayesplot-ppc-ribbon-using-points-for-observations/19599, it'd be nice to have a ppc plot which uses a ribbon for the model predictions but points for the observations.

I propose adding an argument to the function ppc_ribbon and ppc_ribbon_grouped, namely y_is_point which defaults to FALSE (so as to not break backward compatibility). So the signature would be

ppc_ribbon(
  y,
  yrep,
  x = NULL,
  ...,
  prob = 0.5,
  prob_outer = 0.9,
  alpha = 0.33,
  size = 0.25,
  y_is_point = FALSE
)

I can build this feature in.

[jgabry]

Thanks Charles, I'm in favor of this. Regarding the argument, in the ppc_violin_grouped() function we currently have an argument y_draw with three options y_draw="violin", y_draw="points" or y_draw="both". Could we just do the same here but the first option is "line" instead of "violin"?

[charlesm93]

Sure thing. I'll have y_draw default to "line".

[jgabry]

closing this since we merged the PR already