Added navigation controller, removed test buttons and b, c variable buttons;
Rename variable 'a' to 'x'
Move 'x' and 'enter' buttons
Added graph VC. Added segue to graph; Added 'graph' button.
Added GraphViewController and link it with the VC in IB
Added graph view
Removed scaffolding code form FaceView
Moved graphing code into a Graph group
Added AxesDrawer class and use it to draw aces in GraphView
Add scale and origin as properties of Graph view
Methods to convert between view and axes coordinates
Take advantage of retina screen by iterating over pixels
Add GraphDataSrouce protocol. Using delegate in GraphView to calculate Y coordinate for given x coordinate
Added pinch and pan gestures
Handle tripple-tap to move graph origin
Changes in storyboard
Added program as a model to graph VC
Use calculator brain to calculate Y value for graph's X
Fix Y coordinate calculation
Add calculation caching to improve panning/zooming performance