The reference measurement can be selected either from file or from the measurements available in memory.
If you want to select the measurement from file, first check , then click the button to open the browser. Note that the selected file is not added to the memory measurement file list.
If the measurement is selected from memory as shown in the figure above, it is possible to update the reference measurement when a new measurement is performed by checking . See Plot->Plot Type/Routing/Test... on how to set-up the routing.