(Notation below: $y$ is data value, $y_{pred}$ is predicted value, with $y_{pred}$ computed as $stat(y_{rep})$ from posterior draws.)
The usual residual analysis plots $y - y_{pred}$ on the vertical axis with $y_{pred}$ on the horizontal axis.
But ppc_error_scatter_avg() plots $y - y_{pred}$ on the vertical axis with $y$ on the horizontal axis. This is confusing and difficult to interpret (for me, but I'm not alone: https://stats.stackexchange.com/a/146002).
I would be great if ppc_error_scatter_avg() plotted $y_{pred}$ on the horizontal axis, either by default or with that as an option. (Or maybe there's already an easy way to do that; sorry if I missed it.)
Thanks for considering!
P.S. My comment here assumes that $y_{pred}$ is computed as $stat(y_{rep})$ from posterior draws. But the related thread regarding residuals #349 seems to suggest that residuals are computed as $stat(y - y_{rep})$, with $stat(y_{rep})$ not separately, explicitly computed. Hmmm...?