Click the Angle Clockwise or Angle Counterclockwise button on the Formatting toolbar.
PowerPoint angles the labels.
If your axis labels are long, you can rotate them slightly to make them easier to read. For example, if your x-axis labels bump up against each other because of your chart's size or the amount of data it contains, you can angle the labels to make them more legible.
Toggling On or Off
The Angle Clockwise and Angle Counterclockwise buttons toggle the angled labels on or off. To return to regular text, simply click the appropriate angle button again.