Can I add a horizontal average line to a chart without calculating it in cells?

[RockandGrohl]

Hi all, have a table with the months across the X axis and then various cost metrics etc down the Y axis.

To work out the average, I've added a new row to the table and then just done

=AVERAGE($B$7:$H$7)

But to plot this as a horizontal line on the bar chart, I must repeat this average figure across the width of the table.

Is there a way to calculate the average once, and then reference that as a line across the chart? My new calculation cell is in B37

Thanks!

 

[GraH]

You can reference it 12 times, one for each month