You want to show the average of something on a chart, as a horizontal line. That means, you will have to create a column with that average in a table, where the same number is shown in the entire column.
It’s pretty clear, the formula =Average(<N° visitors>) won’t do the trick. It just returns the number itself, because it operates in the context of the month. To solve this, we can use 2 formulas :
Average(<N° visitors>) in Report
Average(<N° visitors>) forall(<Month>)
The first one is absolute, so it will always return the same number – no matter in which table you put it. The second one is relative – it depends on the dimensions already in the table.
One way to do this is by choosing : Data> Define as variable and choosing Evaluate the formula in its context.
Both of the above will return :
Now, we turn this into a chart :
Not exactly what we need, but, getting there.
Rightclick the chart and choose “Format Chart”
On the Series tab, click “Add” and make it a line chart, next, you drag the average from the other group to the new group.
And we get :