Fix warning in matplotlib rendering.

See warning at bottom of:
http://open.cdash.org/viewBuildError.php?type=1&onlydeltap&buildid=2741779

Change-Id: Ib094e2f0d7a8ae6c3a591aadcb853222c598c40f
Merge topic 'FixWarning' into master

322fb5e9 Fix warning in matplotlib rendering.