Give no background color to the pie charts

Currently, we hard-code a white background color for the pie charts.
This however is unneeded (as far as I can tell), and makes things look
ugly whenever the horizon css is changed to not have a white background.

Change-Id: Iac4fc1b9f77bddedc92eb627b22a8add88a0b35d
Merge "Give no background color to the pie charts"