Providing Count of Line Item option will helps to get detail of Unique Line Item Count.
Example: I have Captured Customer ID in a Variable, i only want unique Customer ID Count. There is no Metric like Count in SiteCatalyst.
Currently we have created the calculated Metric to get count detail.
This is possible with the Approximate Count Distinct function: https://www.youtube.com/watch?v=80YYjKfHpHw
Yes.. Currently we did it in the same process. its little bit difficult.
I suggest adobe to provide this feature in Report Builder itself instead of creating calculated metric.