I have a spreadsheet containing data over various numbered weeks. From this I
use VLookup to produce a graph showing the data from the last 13 weeks. Each
time the current week number is updated, the last 13 weeks change in turn.

On the same spreadsheet I also keep the data by month. I now want to use a
similar VLookup to produce a graph showing the last 6 months. How/Is it
possible to give eg November a numerical value so the other rows are eg
November-5 (June) November-4 (July) etc?