A Pareto chart answers "how few items make up most of the total?". It has two parts on a dual axis: bars sorted from largest to smallest, and a line with the running percent of total, which climbs to 100% at the last bar. Where the line crosses 80% tells you how many items carry 80% of the total.
The line is a table calculation: RUNNING_SUM(SUM([Sales])) / TOTAL(SUM([Sales])). Two details decide whether it's right:
- Sort first. A running total adds the bars in the order they appear. Unsorted, the line still ends at 100% but its shape means nothing.
- Direction. Table calculations run Table (across) by default. With the bars across Columns that's right; with the members down Rows, set Compute Using to Table (down).
Then make it a dual axis, keep the bars as bars and set the running percent to a line.