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Right click the graph legend and choose Update Legend... from the context menu. This opens the legendupdate dialog box. When the Auto Legend Translation Mode is set to Custom, the Legend Custom Format control now provides a drop-down populated with strings that include typical metadata elements such as worksheet name, dataset name, 1st user parameter etc, exposing user to available syntax for custom legend annotation. The full reference on the notation is available from the help file on pressing F1.
Legend Substitution Notation for User-defined Parameter Rows (ORG-9221)
In the past, user uses @LD1, @LD2 as substitution notation for user-defined parameter rows. But it's hard for user to figure out which is the index. In Origin 9.1, we extended the notation by using @LD_name where name can be the first part of the UDP name, like @LD_T can look for any that start with T, or @LD_PR to find Pressure.