+ * Paints the background layer of the signal with a QPainter
+ * @param p the QPainter to paint into.
+ * @param left the x-coordinate of the left edge of the signal.
+ * @param right the x-coordinate of the right edge of the signal.
+ **/
+ void paint_back(QPainter &p, int left, int right);
+
+ /**
+ * Paints the mid-layer of the signal with a QPainter