The options to customize the format of an expression label in a chart are quite limited. If you enable the design grid (in the View menu) you can right click an expression header cell and choose the option Custom Format Cell. There you will have limited options to change the text format.
The format of the text of an expression header can be customized using the Custom Format Cell option. If that's not what you want to do then you'll have to explain to me what "this text formats to the expression's label" means.