This sounds loosely related to my question, Visualizing predictive uncertainty. In that case, there were potentially NAs in the y values. The first (left) part of the graph showed the fit of the model. I wanted (and still want) to show on the second (right) part of the graph the forecasted results and their uncertainty (e.g., 50% anyd 90% CIs). There is no y to show, as that future has yet to happen, ppc_intervals() won’t work unless the length of the y values is equal to ncols(newdata), and it won’t let me append NAs to the y values to make them equal.