fix rectangle rendering

I had to disable it at the same time as it renders a white space over
the whole chart right now. Before the fix it was just the top left
quadrant.

Change-Id: I499767bcc302e6d6c82437535b4ea10ba212b82d
fix missing clearing of rendering area

Now there are no artifacts left from earlier renderings.

Change-Id: I48e6585d838cdf9547c150e1484a967fc9b69329