Fix BarChart hovering.
Fix BarChart hovering.

Using margin.right for the width of the white rect was just wrong.
This CL computes the width of the white rect from the widths of the texts that will
go on top of it.

This CL also elides the key text when there's only one series.

Review-Url: https://codereview.chromium.org/2255303002

catapult-project/catapult-csm,sahiljain/catapult,sahiljain/catapult,sahiljain/catapult,catapult-project/catapult-csm,benschmaus/catapult,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult-csm,sahiljain/catapult,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,benschmaus/catapult,sahiljain/catapult,catapult-project/catapult,benschmaus/catapult