statistics: fix placement of horizontal labels in bar charts

The subtraction of half the label width, needed for centered
labels, must have been lost somewhere.

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

stats/barseries.cpp

@@ -151,6 +151,7 @@ void BarSeries::BarLabel::updatePosition(bool horizontal, bool center, const QRe
 				setVisible(false);
 				return;
 			}
+			pos.rx() -= itemSize.width() / 2.0;
 		}
 		item->setPos(roundPos(pos)); // Round to integer to avoid ugly artifacts.
 	}