Uwe Rathmann

avoid isBarVisible lookups for each sample

using if/else for QwtPlotMultiBarChart::boundingRect

no need to iterate and do lookups to know the number of visible bars

using isBarVisible to improve readability

using QSet

unused headers removed

initial implementation

QwtGraphic legend icons render blank on Qt 6.5+ (initial null DirtyTransform collapses painter)