statistics: paint custom grid

With removal of QtCharts' axes, the grid was lost. Readd it.

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

stats/statsgrid.h

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0
+// The background grid of a chart
+
+#include <memory>
+#include <vector>
+#include <QVector>
+#include <QGraphicsLineItem>
+
+class StatsAxis;
+namespace QtCharts {
+	class QChart;
+};
+
+class StatsGrid  {
+public:
+	StatsGrid(QtCharts::QChart *chart, const StatsAxis &xAxis, const StatsAxis &yAxis);
+	void updatePositions();
+private:
+	QtCharts::QChart *chart;
+	const StatsAxis &xAxis, &yAxis;
+	std::vector<std::unique_ptr<QGraphicsLineItem>> lines;
+};