How to show average hours on graph

Here is a working solution for you.  I created a measure that respects the current filter context and displays an average hours line.  I had to convert the hours to a decimal number first 

here is the sample workbook https://www.dropbox.com/s/v8y46cw5bixolam/average%20hours.pbix?dl=1

Here is my measure

Average = CALCULATE(sum(Data[Decimal Hours])/DISTINCTCOUNT(Data[Date]),filter(all(Data[Date]),Data[Date] >=FIRSTDATE(Data[Date]) && Data[Date] <=LASTDATE(Data[Date])))

Here is the hour conversion (PQFL)

= Table.AddColumn(#"Extracted Time", "Decimal Hours", each Time.Hour([hour]) + Time.Minute([hour])/60)