A reference index should be included in a display when the user must compare displayed information with some critical value.
Additional Information:
Indexing may be complicated in some situations, such as when a series of bar charts do not have a common measure. In such a case, it might help to use an index scheme such that bar lengths will fall in the same zone under normal conditions, so that deviations in bar length will be readily noticed by users who must monitor changing data. For example, a horizontal line might be an adequate reference index for a vertical bar graph.