I think your MonthID should be fine for a contigouus axis (if you have a Date type field as base for rolling Month, you could also use monthstart(DateField) to create something similar (to enable crossing the year bounderies).
So what you are trying to plot? You said Growth percentage, but for what date?
I assume you just have a Value Field next to your MonthID field, with a Value for each Month and you want to see the percentage change from Month to Month?
So you could create a linechart, use your MonthID as dimension (or maybe RollingMonth, sorted by MonthID? I think that would be easier to understand for human beings), then use as expression something like:
(Calculating the difference of current Month's value to previous Month's value, devided by previous Month's Value), or something similar.
Then you could limit the max. visible Months to 12, if you want, in presentation tab and enable scrolling.
Maybe like attached.
So, if you want something like this, I think this should be pretty straight forward to adapt.
I realised not long after posting that the real requirement was not actually highlighted. What I need to plot is a %growth marker for the month based on the same month in the year previous - May2011 vs May2010 (not the previous month eg May2011 vs Apr2011)
I thought I could use some form of expession along the lines of =(MonthID-(MonthID-12))/(MonthID-12) but getting this to work in SET is proving more difficult than I expected.
Thank you for your post though... I hadn't come across the 'ABOVE' function yet.
Yes, that's quite hard to do with a set expression, since a set expression is evaluated only once for the complete chart. So IMHO you can't use the current dimension value inside your set expression, which you probably need to use for your calculation, if MonthID is your dimension.
What you could try is e.g. using a data island MonthID field to plot against with some if()-logic or probably better an as-of flag in your script for your MonthIDs, this concept was discussed here several times, e.g. here: