plotter: Remove pivot vals that remain empty after a filter application
plotter: Remove pivot vals that remain empty after a filter application

Change-Id: I0792fae1d92b7300bb84283d0afa6848efaadc4b
Signed-off-by: Kapileshwar Singh <d373e2b6407ea84be359ce4a11e8631121819e79@arm.com>

JaviMerino/trappy,bjackman/trappy,joelagnel/trappy,JaviMerino/trappy,ARM-software/trappy,bjackman/trappy,JaviMerino/trappy,ARM-software/trappy,bjackman/trappy,derkling/trappy,derkling/trappy,ARM-software/trappy,joelagnel/trappy,sinkap/trappy,joelagnel/trappy,derkling/trappy,sinkap/trappy,ARM-software/trappy,sinkap/trappy,joelagnel/trappy,bjackman/trappy,sinkap/trappy