statistics: render regression item using QSGNode

Render the confidence area and the regression line into a pixmap
and show that using a QSGNode.

It is unclear whether it is preferred to do it this way or to
triangulate the confidence area into triangles to be drawn by
the shader.

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

stats/regressionitem.h

@@ -0,0 +1,26 @@
+// A regression line and confidence area
+#ifndef REGRESSION_H
+#define REGRESSION_H
+
+#include "chartitem.h"
+
+class StatsAxis;
+class StatsView;
+
+struct regression_data {
+	double a,b;
+	double res2, r2, sx2, xavg;
+	int n;
+};
+
+class RegressionItem : public ChartPixmapItem {
+public:
+	RegressionItem(StatsView &view, regression_data data, StatsAxis *xAxis, StatsAxis *yAxis);
+	~RegressionItem();
+	void updatePosition();
+private:
+	StatsAxis *xAxis, *yAxis;
+	regression_data reg;
+};
+
+#endif