statistics: add notion of Z-value to chart items

The chart items were drawn in order of creation. To control this,
add a notion of Z-value. In contrast to QGraphicsScene, make
this a small integer value.

To controll order of drawing, a plain QSGNode is created for
every possible Z-Value and items are added to these nodes.
Thus, items are rendered by Z-value and if the Z-value is equal
by order of creation.

Likewise split the list of chart-items into Z-values, so that
items can be quickly unregistered: The items that will be
removed individually will usuall be part of Z-levels with only
few items (e.g. legend, infobox). Z-levels with many items
(notably the series) will always be fully rebuilt.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

1 files changed, 2 insertions, 1 deletions

diff --git a/stats/informationbox.cpp b/stats/informationbox.cpp
index c768b77b3..02fdb9e97 100644
--- a/stats/informationbox.cpp
+++ b/stats/informationbox.cpp
@@ -12,7 +12,8 @@ static const double informationBorderRadius = 4.0; // Radius of rounded corners
 static const int distanceFromPointer = 10; // Distance to place box from mouse pointer or scatter item
 
 InformationBox::InformationBox(StatsView &v) :
-	ChartRectItem(v, QPen(informationBorderColor, informationBorder),
+	ChartRectItem(v, ChartZValue::InformationBox,
+		      QPen(informationBorderColor, informationBorder),
 		      QBrush(informationColor), informationBorderRadius)
 {
 }