By default, the number format displayed on the forecast chart comes from the underlying format of the forecast cell. You can select another cell format using the Chart Preferences dialog.
To change the format of numbers in a forecast chart:
Choose a format from the drop-down list. Available formats are similar to Microsoft Excel formats. For most formats, you can specify the number of decimal places and whether to use a thousands separator.
Click OK to apply the settings, or use Apply To as described in Applying Settings to the Current Chart and Other Charts.