Display on the same chart sales from one month purshases from previous month

[philpelll]

Hello, 

I'm looking for some help because I'd like to display on the same line chart : 

- Sales of Month M

- Purchases of Previous Month M-1

For that, I've created a dimension Month_Previous = AddMonths(Month,-1)

Then, I though that a measure as followed would have worked : Purchases_Previous_Month = Aggr(sum(Purchases),Month_Previous), if I display the line chart as followed : dimension equals to Month and Measures equal to Sales and Purshases_Previous_Month

But Purchases_Previous_Month displays Purchases. I guess it is a matter of context but I don't know how to handle it. 

Hope someone would help me with it !

Thank you !

[dplr-rn]

whats the dimensions you  have.

typically you do it like so

your dimension

2 measures 

current month - Sum(  {<MonthYear={'$(=Max(MonthYear))'}>}  LineSalesAmount)

previous month - Sum(  {<MonthYear={'$(=Date(AddMonths(Max(MonthYear),-1),'MMM-YYYY'))'}>}  LineSalesAmount)