Dynamic Graph with Text categories

I am wanting to create a 6 month dynamic graph, where upon the changing of month in the data validation cell, the graph will change the furthest right bar to be the current month, and each bar, the subsequent previous month, up to the last 6 months.
I am able to do this for weeks by linking the cells, then saying e.g. "G9-1" and the week prior will be "Current week -1", then the VLOOKUP would grab the Sales number. However with Text, you can't simply do "G9-1" as months won't calculate.

Could anyone enlighten me on a way to do this?

Thanks!