The default settings for Data Labels attached to graphs can cause issues with overlapping, which in turn reduces readability.
In order to rotate the labels, select your graph, then select a single data label (each of them will be highlighted and affected by any changes). Open the properties window whilst the data labels are selected, and expand the “Labels” title, then expand the “Label” tag in the Labels category. You’ll be presented with the below properties:
As you may have guessed, the “Rotation” attribute controls the angle of the label. By default, this is set to zero, which will give you horizontal text.
A value of -90 to 90 can be selected. Any number below zero will swing the end of your text upwards, while any number above zero will tilt the end of your text downwards. The below is the result of using the above properties (-90 degrees):
