2D: Optimize barmodelmapper


Previouly, the QBarmodelMapper initialized the whole model when the
series was changed.

[ChangeLog][Graphs2D] The QBarModelMapper doesn't reinitialize the model
every time something changes.

Pick-to: 6.12
Task-number: QTBUG-151151
Change-Id: Ie4b06aa633eafbaa5f3f9877bf9728cd767e6539
Reviewed-by: Tomi Korpipaa <tomi.korpipaa@qt.io>
commit0ca1a97f1a903061f928b36a92a4925323c96504
parent1c7e8cacaced15db99c890a2f78599fb58b3c234
authorSami Varanka <sami.varanka@qt.io>
committerSami Varanka <sami.varanka@qt.io>
commit timeTue, 13 Oct 2026 06:49:20 +0000