Add Static value to cumulative data

[Qlik007]

three columns 

date                                   Sales             production cost        

jan 2022                          5 Mill               4 Mill

feb 2022                         2.4 Mill              6 Mill

April 2022                       4 Mill                 8 Mill

May 2022                          5 Mill               4 Mill

graph shows accumulated value of Sales ( sum of sales with modifier set to "Accumulation " ) 

what i want to show 

sum of sales with modifier set to "Accumulation "  which adds the amount of previous month production cost  

so for the month of May i want to see total of 5 mill (current month value ) + 8 mill ( previous month's value) 

currently  values goes to null for previous month's sales 

so this is what i t looks like 

date                                   Sales             production cost        

jan 2022                          0 Mill               4 Mill

feb 2022                         0 Mill              6 Mill

April 2022                       0Mill                 8 Mill

May 2022                         5 Mill               4 Mill

i am using this expression for Sales measure  "=if (sum(sales)=0, null(), Sum(sales)) "

so that on graph the line starts from may ( which should show 5 plus 8(previous month production cost) mill 

Can you guys help me