I erased the Y-axis title. How can I put it back?

Prism 5:

1. Double click on any axis to bring  up the Format Axis dialog.
2. Go to the TItles & Fonts tab.
3. Check the option box(es) to show the title(s) you want to bring back
4. OK from the dialog.
5. Edit the generic titles right on the graph.

Older versions:

 

1. Double-click in the area where the Y axis title belongs. When you hit the right spot, you'll see a text insertion cursor, and you can start typing. If you pull down the "Edit" menu and choose "Select... All", the area to click on will be shaded.

    

2. Create an independent text object by clicking on the text tool. Then click on the graph where you want the text to start and type. Click elsewhere on the graph to end the text entry. Then click on the new text, and choose "Format.. Text" (ver 3) or "Change... Selected Text..." (ver 4). The rotation choice on that dialog lets you make the text vertical.