Show average line by series on a line chart

I have a line chart to show actual percentages by month for three different
values I am tracking, I would like to show the average for each item as well
on this chart as a line across the chart so that for each month one can see
if the actual percent is above or below the average for the year, is there a
way to do this easily, I have a cell from the data table that shows overall
year average for each value.
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Judy Rose Cohen
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