Display average value in chart

I calculated the average value in aggregate and I would like to put this value in legend of a linechart. How can I do this?

Been there. 

Not sure if it is the only solution, but what I do is to add an extra series (average) with the same points as the other lines, but always with the same value, in order to achieve something like the image below.

Hope it helps!

It is a good solution but I have to put the average in a legend, not inside of the chart.

It is impossible to put the average in a legend?