Line + Band with variables that Band does not support is awkward

[mwaskom]

Band does not support linestyle, so this plot is wrong in a confusing way:

(
    so.Plot(fmri, "timepoint", "signal", color="region", linestyle="event")
    .add(so.Line(), so.Agg())
    .add(so.Band(), so.Est())
)

One needs to do this:

(
    so.Plot(fmri, "timepoint", "signal", color="region", linestyle="event")
    .add(so.Line(), so.Agg())
    .add(so.Band(), so.Est(), group="event")
)

Perhaps the stat grouping should use any variables defined at the common level or in that layer, not just those the mark accepts?

This will have some implications as we address #2911