How to add an "Average" line in a simple line chart

Vertical or horizontal lines can also be added as error bars emanating from any single point.  In this case, the average could be plotted at the left of the plot and a fixed value of the chart width chosen for the positive horizontal error bar.  I tend to match the colour to the data series but show averages, maxima and minima as some form of dashed line.

There are number of options.  one option is just add a column/row next to the data that for all months = average of the months and then add that column/row to the graph.  you can add the data either on the graph change the data range or insert additional data or you can copy the data range on the sheet and then on the graph use paste special.

Just note, although not the average line you could also consider adding a trendline for the data series.