Either horizontal or vertical bars can be used to display data that are grouped into categories or a small number of time intervals. However, there are data relationships where one orientation is preferable to the other:

• Vertical bars should always be used when they represent data over time.
• Horizontal bars should be considered when the labels for groups or categories do not fit easily in a horizontal alignment along the x axis.

If you are including bar graphs for both time-series data and categorical data, consider reserving vertical bars for the former and horizontal bars for the latter. This differentiates these data types, and provides a consistent association between data relationships and graph type, which will assist reader understanding.