statistics: implement a legend box

For some chart (e.g. pie charts or stacked bar charts), we want
to display a legend. QtCharts' legend interface happens to be
private and therefore is of no use.

This introduces a legend box which is implemented using
QGraphicItems, which can be placed on top of QCharts. It's very
unfancy, but works for now. If there are too many items, not
all are shown. Currently, the legend is configured to fill
at most half of the width and half of the height of the chart.
This might need some optimization.

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

1 files changed, 36 insertions, 0 deletions

diff --git a/stats/legend.h b/stats/legend.h
new file mode 100644
index 000000000..e6c5af1e0
--- /dev/null
+++ b/stats/legend.h
@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: GPL-2.0
+// A legend box, which is shown on the chart.
+#ifndef STATS_LEGEND_H
+#define STATS_LEGEND_H
+
+#include <memory>
+#include <vector>
+#include <QGraphicsRectItem>
+
+class QGraphicsSceneMouseEvent;
+
+class Legend : public QGraphicsRectItem {
+public:
+	Legend(QGraphicsWidget *chart, const std::vector<QString> &names);
+	void hover(QPointF pos);
+	void resize(); // called when the chart size changes.
+private:
+	// Each entry is a text besides a rectangle showing the color
+	struct Entry {
+		std::unique_ptr<QGraphicsRectItem> rect;
+		std::unique_ptr<QGraphicsSimpleTextItem> text;
+		QPointF pos;
+		double width;
+		Entry(const QString &name, int idx, int numBins, QGraphicsItem *parent);
+	};
+	QGraphicsWidget *chart;
+	int displayedItems;
+	double width;
+	double height;
+	int fontHeight;
+	std::vector<Entry> entries;
+	void updatePosition();
+	void hide();
+};
+
+#endif