Display mean value in chart legend

I generated a line chart in SSDT 12 using the following query:

SELECT FCP, NFS, TimeIndex
FROM Controllers
WHERE ControllerName LIKE 'Controller-A'

Then I created a trend line using the guide here. In step 5 of the guide it says to set the legend text to 'Average'. How do you set it to display the mean value?

Thanks in advance
Adam