plotview: Hook up zoomLevel & fix up scrolling

spectrogramplot.cpp

@@ -118,7 +118,7 @@ void SpectrogramPlot::mousePressEvent(QMouseEvent *event)
 void SpectrogramPlot::paintMid(QPainter &painter, QRect &rect, range_t<off_t> sampleRange)
 {
     for (int x = rect.left(); x < rect.right(); x += linesPerTile()) {
-        QPixmap *tile = getPixmapTile(x * fftSize);
+        QPixmap *tile = getPixmapTile(sampleRange.minimum + x * getStride());
         painter.drawPixmap(QRect(x, rect.y(), linesPerTile(), fftSize), *tile);
     }
 }