OK, found it. The xy-buffer is not intended to buffer multiple plots, just multiple points for a single plot. The array input is used if there are multiple points added at once to a single plot.
What you need to do is buffer each plot separately, then build the array for the multi-plot using "build cluster array". See e.g. XY-Chart C or D in the quoted example. The XY-buffer is reentrant, thus can be used multiple times, each with its own data.
