I have a stacked bar chart with three series, and I'd like to connect
categories across all three series using series lines. One of the
series has an empty category, which I'm placing as the last (top) item
on each stacked bar.

There is a nice way to manage how line charts handle missing data, but
series lines don't seem to behave the same - they are all either on or
off. Is there way to use series lines but force them to leave gaps when
plotting the empty cell?