add override options to allow multiple stacks

Flot allows to group series in multiple stacks by setting their _stack_ value to the same key (_true_, number or string, see https://github.com/flot/flot/blob/master/jquery.flot.stack.js#L13 for reference).

Unfortunately I don't have a Graphite instance available to test the PNG graphs, so this is only tested with the Flot backend.