I have an expression set up that lets me average the amount of days it takes to complete a project. Within the dashboard I have a Multibox set up that allows the use to select the month they would like to view, which filters the below to only displayed completed stages within that period of time.
This is expressed as a bar graph, and ideally I would like to create a new expression that looks the previous month than what is currently selected.
I guess in your set analysis you can include one more condition if you have a date dimension already. In the below expression, I am also assuming that you have months in your date dimension such as Aug, but formatted as date:
Is your MonthName field formatted as date or text? If the month and year names are aligned to the left, then they are text. if left, then they are dates. The above formula works only if your field is formatted as date.