In#343,@TeemuSailynoja highlighted the lack of residual plots, especially for discrete observations. It is about the fact thatppc_error_scatter_vs_x scatter doesn't work for discrete observations, andppc_error_binned doesn't currently support covariates on the x-axis. I think, as he suggested, adding an optionalx argument toppc_error_binned, which would work similarly toppc_interval, makes sense. That way, users will have the ability to plot residuals againstx, but since it's optional, this change won't break any existing plots.

Another point regarding residual plots is having a new plot -possibly- namedppc_residual suggested by@jgabry and@avehtari at#349. That new function would ploty - stat(y_rep) on the y-axis andstat(y_rep) on the x-axis. This, again, is a good new plot to implement in my opinion since it gives users a chance to analyse a different aspect of the data.

I am willing to work on both of these functions and more if there is more to update regarding residual plots, however, I am not sure where to start since there are open PRs connected to residual plots.