Graph Settings

Hit the "Set" button in the graphing window to open a dialog where the axis limits can be set precisely:

Tap the "x" symbol in the gray circle to erase the current value and enter a new one. You must enter a valid numerical value (scientific notation is OK) in order to dismiss the keyboard! There is no need to press "Done" while switching from one text field to another - "Done" should be pressed at the end of editing.