parameterize and make optional the series chart sorts

we assume we're dealing with a group keyed on key-pairs, which doesn't
sort properly.  sorting is more efficient and convenient as part of the
nest operation, so do it there based on this assumption...
but also allow the user to override or disable this functionality
partial progress on incremental series charts
allow legend to rerender

render method removes old legend
more dramatic test of incremental changes to series chart
rerender legend if series set changes