Only display equation for graph if non-empty.
Set GraphViewController as dataSource delegate for GraphView.
Implement drawRect: for GraphView.
Add AxesDrawer at scale 1.0.
Enable graph scaling, and control of scale with pinch gesture.
Add pan gesture to move origin.
Add triple-tap gesture to move origin.
Use NSUserDefaults to preserve user-defined origin and scale, add Reset button.
Set GraphView to UIViewContentModeRedraw.
Convert to Universal application. Add separate iPad storyboard, with Graph button set up through target-action.
Set GraphViewController as UISplitViewControllerDelegate.

Set and remove barButtonItem as needed.