To add an average line to your chart (like the one in the example above), follow these steps:
With your visual selected, go to the Custom tab in the design panel and click the ‘+’ icon next to Marks to add a new mark.
Set the mark type to Line.
Drag the relevant field into the X axis column (e.g., Date).
In the Y axis column, double-click to add a calculation. Enter the formula:
AVG(Value) OVER ()(Replace "Value" with the field you want to calculate the average of.)
Click here to view the canvas with this example.