How can I re-enable the y-axis on the following line graph?
I have tried right clicking various places with no success.
Right click on the field in the rows shelf and select "show header"
If you have hidden an axis or a header in the view, you can show it again from its related field in the view.
To hide an axis
Right-click (control-click on Mac) the axis in the view, and then clear the check mark next to the Show Header option.
To show a previously hidden axis
Right-click (control-click on Mac) the continuous field in the view that is associated with axis you want to show, click its drop down menu, and then click Show Header.
For related details, see Axes in Parts of the View.