A reference line gives every bar something to be compared with: the average, a target, last year's number.
The quickest way is the Analytics pane, next to the Data pane: drag Average Line onto the view. For more control, right-click an axis and choose Add Reference Line.... The dialog has tabs for a Line, a Band (shading between two values) and a Distribution (shading between percentiles).
Two settings matter most:
- Value: which computation (Average, Median, Minimum, Maximum, Sum) or a Constant you type, such as a sales target.
- Scope: Entire Table draws one line across everything; Per Pane draws one line per section of the view, for example one average for each category; Per Cell draws one per cell.
The line is computed from the marks you see, so it changes with filters.