Assuming your data look like this
Table
+------+----------+-------+
| week | sales_ly | sales |
+------+----------+-------+
| 1 | 65 | 100 |
+------+----------+-------+
| 2 | 93 | 130 |
+------+----------+-------+
| 3 | 83 | 134 |
+------+----------+-------+
| 4 | 3083 | 59 |
+------+----------+-------+
| 5 | 30984 | 39 |
+------+----------+-------+
| 6 | 38 | 580 |
+------+----------+-------+
| 7 | 28 | 94 |
+------+----------+-------+
| 8 | 48 | 93 |
+------+----------+-------+
| 9 | 24 | 984 |
+------+----------+-------+
| 10 | 49 | 95 |
+------+----------+-------+
You need to create two cumulatives and sum them in the same measure.
Rolling Sum =
VAR CurrentYearCumulative =
CALCULATE(
SUM('Table'[sales]),
FILTER(ALLSELECTED('Table'),'Table'[week] <= MAX('Table'[week] ) )
)
VAR LastYearCumulative =
CALCULATE(
SUM('Table'[sales_ly]),
FILTER(ALLSELECTED('Table'),'Table'[week] >= MAX('Table'[week]) )
)
RETURN
CurrentYearCumulative + LastYearCumulative
The output
