sum(Measure)/sum(Total <Dimension2> Measure)
Instead of using Dimension1, I used an AsOf table, as John Witherspoon http://community.qlik.com/people/johnw suggests - that meant I didn't need to use accumulation in the chart.
The only odd thing is that on my client (QV11) John's script for generating an AsOf table didn't work. I've duplicated the script from here http://community.qlik.com/message/89017#89017 and it only works if I add a second dimension with the same values into the load. Any ideas why?
(ooxml, embedded labels, table is Sheet1);
LOAD DISTINCT Dimension, Dimension AS Dimension1
LEFT JOIN (AsOf)
LOAD Dimension1 as AsOfDimension1
INNER JOIN (AsOf)
,if(AsOfDimension1=Dimension1,'Current','AllPrevious') as Dimension1Type