libs/utils/trace_analysis: allows to plot even just a single signal
libs/utils/trace_analysis: allows to plot even just a single signal

A single signal can still be interesting to represent, fix the checks to
enable this kind of plots.

Signed-off-by: Patrick Bellasi <431c86f368affbc3148c4669fbd465131f7e82cb@arm.com>

ARM-software/lisa,mdigiorgio/lisa,joelagnel/lisa,joelagnel/lisa,ARM-software/lisa,credp/lisa,JaviMerino/lisa,arnoldlu/lisa,ARM-software/lisa,credp/lisa,credp/lisa,bjackman/lisa,mdigiorgio/lisa,arnoldlu/lisa,ARM-software/lisa,bjackman/lisa,credp/lisa