Explanation/Reference:
Example (see step 4 and 5 below): Getting Top-N Sales Reps for Year and Year-1
1. Under the "Business Model and Mapping" Layer, right click the "F1 Revenue" and select "New Object" ->
"Logical Column..."
2. In the "Logical Column" Window give for Name: Year Ago Revenue as seen below:

3. Goto the "Column Source" Tab and select "Derived from existing columns using an expression" radio button then click on the "Edit Expression" icon.
4. Enter the following expression in the "Expression Builder" Window by selecting "Functions" in the
"Category:" list, "Time Series Functions" in the "Functions:" list then "Ago" "Time Series Functions:" section.
5. Select the "<<Measure>>" in the "Ago( )" function, then select "Logical Tables" in the "Category:" list, "F1 Revenue" in the "Logical Tables:" list, then double click the "Revenue" in the "Columns:" list.
Etc.