Optimize barcharts painting

Previously, bar charts used to recreate the bar chart visuals from
scratch whenever any change to the bar data occurred.
The following changes were made:
- Graphics items are generated only for visible bars and labels
- Graphics items are reused if layout or data changes
- Labels are not generated unless they are visible
- Only newly added bars are animated from scratch; existing bars
  will retain their size, making things more visually pleasing.

With these optimizations, the size of the bar series is irrelevant
to the painting performance, only thing that matters is how many
bars and labels are actually within visible range.

Task-number: QTBUG-52442
Change-Id: If3199a86e6819ef6219d2eb5f06d729c0bdbd2ae
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>

0 files changed, 0 insertions, 0 deletions