plot: Draw centre-line

plot.cpp

@@ -17,4 +17,13 @@
  *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#include "plot.h"
+#include "plot.h"
+
+void Plot::paintBack(QPainter &painter, QRect &rect, range_t<off_t> sampleRange)
+{
+    painter.save();
+    QPen pen(Qt::white, 1, Qt::DashLine);
+    painter.setPen(pen);
+    painter.drawLine(rect.left(), rect.center().y(), rect.right(), rect.center().y());
+    painter.restore();
+}