you can't do this by set analysis. The idea of set analysis is to modify data set which is taken into account for chart calculation. This means that the set analysis is evaluated before chart is calculated. During chart calculation the data is sliced by dimension values. As a result, you cannot add a new dimension value or "show data for previous month on a line for this month" using set analysis. You need different approach.
In your case I would try to create an as-of calendar table in script linking each month to itself plus a "Total" month, which will result in a new dimension containing 12 months plus a "Total" month. This dimension than can be used in your bar chart. You can read more about as-of calendar tables here: The As-Of Table