pmchart: resolve y-axis scaling issue for util charts
pmchart: resolve y-axis scaling issue for util charts

Issue arrived with the Qt5/Qwt6.1 update, where auto-scaling
was being circumvented due to presence of qNaN values in the
dataset.  QwtPlot::updateAxes() in particular was affected by
these, ultimately traversing inValid axis code paths where it
should not have.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp