I'm not sure how to do it with the choices in the chart; i Typically would use ExecuteSQL to generate the data in the chart.
For the "mean" data series, I'd do a SQL statement:
ExecuteSQL ( "SELECT AVG (TotalCount) FROM Colors ";"";"" )
(this is my test file).
This generates the line all the way across the chart as you are requesting.
Thanks. Unfortunately I don't have any experience with SQL.
If you are using found set as data source, you may be able to get grand summary of mean count in Y-Axis as
GetSummary ( Census::Mean Count ; Census::Mean Count )