[graph] Fix a bug when zooming

When entering the Navigate menu, a portion of the Y axis is memoized to
preserve the grid units.
However, this portion was not modified when zooming, causing some
glitches.

Change-Id: I06b4ee044eaa75d48b8f338c9ef6ea33bb299e39

apps/shared/interactive_curve_view_range.cpp

@@ -115,6 +115,7 @@ void InteractiveCurveViewRange::zoom(float ratio, float x, float y) {
     m_yRange.setMax(newYMax, k_lowerMaxFloat, k_upperMaxFloat);
     MemoizedCurveViewRange::protectedSetYMin(newYMin, k_lowerMaxFloat, k_upperMaxFloat);
   }
+  m_offscreenYAxis *= ratio;
 }
 
 void InteractiveCurveViewRange::panWithVector(float x, float y) {

apps/shared/interactive_curve_view_range.h

@@ -14,6 +14,7 @@ public:
   InteractiveCurveViewRange(InteractiveCurveViewRangeDelegate * delegate = nullptr) :
     MemoizedCurveViewRange(),
     m_delegate(nullptr),
+    m_offscreenYAxis(0.f),
     m_zoomAuto(true),
     m_zoomNormalize(false)
   {