Average in chart

[Pejo]

Hi,

I have a report that is grouped by client and datetime and looks something like this

Client A
2001-01-01 08:00 10% (1/10)
2001-01-01 09:00 20% (2/10)
2001-01-01 10:00 50% (4/8)

Client B
2001-01-01 08:00 25% (1/4)
2001-01-01 09:00 33% (3/9)
2001-01-01 10:00 20% (12/60)

...

The second column is incoming calls/total number of calls as I show in parenthesis (this isn't showed in the report though).

Now I try to add a barchart that shows "total average" of incoming calls per datetime for all the clients, ie total number of incoming calls for each datetime divided by total number of calls for each datetime.
The first bar is for 2001-01-01 08:00 and should show 14% (1+1/10+4). The second, 2001-01-01 09:00 and 26% (2+3/10+9) and so on.
The problem is that I can't specify this average formula in the chart. I can get the chart to show the datetime properly on the x-axis and I can get it to show the correct sums of incoming and total calls as bars per datetime, but when I try to show the average between these two it doesn't work.
Does anybody know if it's doable at all and if so, how?


/PeJo