Expression
With this expression, you can modify the legend text. The field is empty by default. If you do not provide any expression here, the legend will display either default labels or the values resulting from the series expression. Whether default labels or expression results are used depends on the status of the Show Series Expression check box in the Label page of the Data Value Properties.
If you provide an expression, use an equivalent identifier as in the series group expression (for example, if you group the series by the compound name, you should also use a series label expression based on compounds and not on samples or injections).