Crystal reports - charting a variable to simulate a running total

Question

I have 2 sets of data as below :
units
costs
What I am trying to do is generate a chart that for each month, reports the result of the formula
sum of costs to date / sum of units to date
Ideally, I would create 2 running totals
RT costs
RT units
then a formula avg. cost to date = RT costs/ RT units
and then for each month (x axis of the chart) plot avg/ cost to date
BUT - crystal won't plot a data set based on running totals.
So, I created a field with a psudo running total
i.e.
whileprintingrecords;
numbervar unitsum;
unitsum:=unitsum+units
and similar for the costs.  Variable called costsum.
Then a variable field
whileprintingrecords;
numbervar results;
results:=costsum/unitsum
So I though I could then plot the field with the variable results in it in the chart.  But no, I cannot see the field name in the data select list.
Please advise how I can create my chart.
Thanks
Edited by: newbyr on Jul 7, 2010 12:20 PM

Answer 1

With the best will in the world, I realise that charting does not like running totals,  What I am looking for is a workaround, or clever way of circumventing the problem.
Before posting I did extensively search the forum and saw the link that DebiHerbert has suggested, and it is of no help at all.
Thanks
Edited by: newbyr on Jul 9, 2010 2:40 PM