How do I orient my Y axis title horizontally?

Prism 4: Double-click on the Y axis to bring up the Format Axis dialog set to the Y axis tab. At the bottom of the dialog, on the right side, you can choose to make the Y axis title go horizontally, or keep it vertical but in the other direction.

Earlier versions: Prism (up to version 3) only lets you make the Y-axis titles vertical. If you want a horizontal label, you'll need to delete the title, then create a separate text object.

To remove the default Y axis title: drag over the title to select it, then press the "delete" key. Click elsewhere on the graph to release the text insertion mode. The title is not actually gone, it is just devoid of content.

Now write the title you want using the text tool. Click on the text tool button,

then click where you want this text to go and type the title. The default orientation of text generated this way is horizontal. You may need to bold and/or resize the text to get it to match the X axis title. Again, click elsewhere on the graph to release the text insertion mode. If the title isn't where you want it, click once to select it, then drag it into position.