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