statistics: remember position of legend when resizing

The position of the legend was reset when resizing. This was
OK as long as the legend wasn't movable.

To avoid resetting the position, store the center position
of the legend relatively to the size of the canvas. On
resize restore the center to the same relative size.

To avoid code duplication, move the sanitizing of the
coordinates from the StatsView to the Legend.

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

stats/legend.cpp

@@ -3,6 +3,7 @@
 #include "statscolors.h"
 #include "zvalues.h"
 
+#include <cmath>
 #include <QFontMetrics>
 #include <QPen>
 
@@ -18,7 +19,8 @@ Legend::Legend(StatsView &view, const std::vector<QString> &names) :
 	ChartRectItem(view, ChartZValue::Legend,
 		      QPen(legendBorderColor, legendBorderSize), QBrush(legendColor), legendBoxBorderRadius),
 	displayedItems(0), width(0.0), height(0.0),
-	font(QFont())	// Make configurable
+	font(QFont()),	// Make configurable
+	posInitialized(false)
 {
 	entries.reserve(names.size());
 	QFontMetrics fm(font);
@@ -103,7 +105,38 @@ void Legend::resize()
 		painter->drawText(rect, entries[i].name);
 	}
 
-	// For now, place the legend in the top right corner.
-	QPointF pos(size.width() - width - 10.0, 10.0);
-	setPos(pos);
+	if (!posInitialized) {
+		// At first, place in top right corner
+		setPos(QPointF(size.width() - width - 10.0, 10.0));
+		posInitialized = true;
+	} else {
+		// Try to keep relative position with what it was before
+		setPos(QPointF(size.width() * centerPos.x() - width / 2.0,
+			       size.height() * centerPos.y() - height / 2.0));
+	}
+}
+
+void Legend::setPos(QPointF newPos)
+{
+	// Round the position to integers or horrible artifacts appear (at least on desktop)
+	QPointF posInt(round(newPos.x()), round(newPos.y()));
+
+	// Make sure that the center is inside the drawing area,
+	// so that the user can't lose the legend.
+	QSizeF size = sceneSize();
+	if (size.width() < 1.0 || size.height() < 1.0)
+		return;
+	double widthHalf = floor(width / 2.0);
+	double heightHalf = floor(height / 2.0);
+	QPointF sanitizedPos(std::clamp(posInt.x(), -widthHalf, size.width() - widthHalf - 1.0),
+			     std::clamp(posInt.y(), -heightHalf, size.height() - heightHalf - 1.0));
+
+	// Set position
+	ChartRectItem::setPos(sanitizedPos);
+
+	// Remember relative position of center for next time
+	QPointF centerPosAbsolute(sanitizedPos.x() + width / 2.0,
+				  sanitizedPos.y() + height / 2.0);
+	centerPos = QPointF(centerPosAbsolute.x() / size.width(),
+			    centerPosAbsolute.y() / size.height());
 }

stats/legend.h

@@ -15,6 +15,7 @@ class Legend : public ChartRectItem {
 public:
 	Legend(StatsView &view, const std::vector<QString> &names);
 	void resize(); // called when the chart size changes.
+	void setPos(QPointF pos); // Attention: not virtual - always call on this class.
 private:
 	// Each entry is a text besides a rectangle showing the color
 	struct Entry {
@@ -28,6 +29,10 @@ private:
 	double width;
 	double height;
 	QFont font;
+	// The position is specified with respect to the center and in relative terms
+	// with respect to the canvas.
+	QPointF centerPos;
+	bool posInitialized;
 	int fontHeight;
 	std::vector<Entry> entries;
 	void hide();

stats/statsview.cpp

@@ -236,14 +236,7 @@ void StatsView::mouseMoveEvent(QMouseEvent *event)
 	QSizeF sceneSize = size();
 	if (sceneSize.width() <= 1.0 || sceneSize.height() <= 1.0)
 		return;
-	QPointF pos = event->pos() - dragStartMouse + dragStartItem;;
-	QSizeF itemSize = draggedItem->getRect().size();
-	double widthHalf = floor(itemSize.width() / 2);
-	double heightHalf = floor(itemSize.height() / 2);
-	QSizeF itemSizeHalf(floor(itemSize.width() / 2), floor(itemSize.height() / 2));
-	QPointF sanitizedPos(std::clamp(pos.x(), -widthHalf, sceneSize.width() - widthHalf - 1.0),
-			     std::clamp(pos.y(), -heightHalf, sceneSize.height() - heightHalf - 1.0));
-	draggedItem->setPos(sanitizedPos);
+	draggedItem->setPos(event->pos() - dragStartMouse + dragStartItem);
 	update();
 }