SSRS 2008 R2 Line Charting y-axis mm:ss

I have the following expression below that I placed in the x-axis:

Int(Fields!AHT.Value/60) & ":" & Format(Int(((Fields!AHT.Value/60)-Int(Fields!AHT.Value/60))*60), "00")

I am haveing a difficult time to get the y axis to format in mm:ss.

Does anyone have an idea or solution how to get the y-axis to read mm:ss?