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

This is the general idea for the data arrangement. Then insert chart | combo chart.

The simplest method is to add the annual average as a second data series. A trendline is not the best choice here, because it shows a trend rather than one fixed average value.

For example, if the months are in A2:A13 and the monthly values are in B2:B13:

1. Add a new column with a heading such as Annual Average.
2. In the first cell of that column, enter: =AVERAGE($B$2:$B$13)
3. Copy the formula down for all 12 months. Each cell in the new column should show the same result.
4. Select the chart, then either expand its source range to include the new column or choose Chart Design > Select Data > Add and select the new average-value range.
5. Format the new series as a line with no markers. A contrasting colour or dashed line usually makes it clear that this is the annual average.

Because the average value is repeated for every month, Excel plots it as a straight horizontal line across the entire chart. The formula also updates automatically if any monthly value changes.

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.