I can't find the correct menu, command, toolbar button, tab, or dialog-box option.

The Microsoft Graph menus and toolbars appear when you double-click a Microsoft Graph chart, or when you click Object on the Insert menu and specify a Microsoft Graph object.
The name of the Format menu command for a selected chart item depends on the item selected. For example, if a data series is selected, the first command on the Format menu is Selected Data Series. You can also double-click a chart item to see the formatting options for the item.
Some commands are available only when the datasheet is active. To switch between the datasheet and the chart, click View Datasheet .