plotter: LinePlot: Fix the legend rendering for missing constraints
plotter: LinePlot: Fix the legend rendering for missing constraints

This fixes the incorrect rendering of the legend when one
or more constraints are missing

Change-Id: Iaacc58c6ec75c93b527c5742dfb759692094f924
Signed-off-by: Kapileshwar Singh <d373e2b6407ea84be359ce4a11e8631121819e79@arm.com>

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