You can override the default label properties.
By default, chart labels use the name of the underlying data item label. You can change the label text to make it more meaningful. For example, you can rename a chart item labeled ISO_3_Letter_Code to Country.
Click the chart node member or measure.
In the Chart Labels section of the Properties pane, set the Custom Label property to Show.
A new chart text item appears.
Do one of the following:
To change the text of the label, double-click the chart text item and type the text.
To change the source of the label text, select the new chart text item. In the Text Source section of the Properties pane, set the Source Type property.
If nothing is specified in the new chart text item, the label appears blank.
Tip: To remove a custom label, set the Custom Label property to Hide.