[apps/sequence] Fix method that should be virtual pure

The following scenaro crashed in DEBUG=1 and did not resize the window
correctly in DEBUG=0.
Got to sequence
u(n)=n
Go to sequence/graph
Remove yAuto
Go to sequence/list
v(n) = cos(n)
remove u(n)
Go to sequence/graph

apps/sequence/graph/graph_controller.cpp

@@ -29,6 +29,10 @@ TermSumController * GraphController::termSumController() {
   return &m_termSumController;
 }
 
+int GraphController::numberOfCurves() const {
+  return m_sequenceStore->numberOfModels();
+}
+
 BannerView * GraphController::bannerView() {
   return &m_bannerView;
 }