Average line based on expression values instead of straight reference line

[lailarhc]

I have a line chart like this:

Where Dimension 1 is a drill-down dimension with Year and month.

Dimension 2 is the name of each employee:

=NomeEmpregado

And the measure represents how many courses each employee has taken:
=count( distinct {<Tipo={'Turmas'}, lotatual={"000075"} >+< lotatual={"00075*"}>} if(IsNull(DataSaidaEmpregado) and (Not isNull(CodEvento)), CodEvento & '|' & CodEmpregado))

I need to add an average permanent line based on an expression value, not a straight reference line.

This is the line graph with the average courses per employee each year:

I want to add this average line to my first graph, so that I have this average line as a permanent feature, no matter the selection. It needs to be always there so I can compare how many courses an employee has taken against the average numbers of courses per employee in a given period of time.

How can I accomplish this?