With the same argument as for DivePercentageItem, move access
to live data out of the paint() function. Instead, calculate
colors in replot(), where the other data are calculated.
This is slightly more complicated than in DivePercentageItem,
since there are multiple polygons. Therefore, replace QPolygonF
by a vector of structures contained the position and color
of the data point.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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