X axis - Display 2 labels on a line graph

[vgadang1]

Hi 

I have a line graph with datedetail as dimension (x-axis) and a measure. I have to display both date detail and fiscal month on the x axis  

Here is the expression I have for the date dimension

 =Aggr(
({{datedetail}})&' '& ({{FY_MONTH}}),
{{datedetail}}
)

I am getting invalid dimension and it doesn't work. 

Thanks,

Vidya