This dialog is available from the Graphs / Options / Legend Text... menu command from the Graphs window.
The legend text displayed in the upper left corner of each graph type can be customized using this dialog. Enter any text you want displayed, and optionally make use of the available data fields. Data fields are displayed in double square brackets, and will be replaced with the appropriate data value when actually displayed on the graph's legend. A wide variety of data fields are available, and can be inserted by pressing the Insert Field... button.