Modify Summary field used for group chart

We have a report developed for drilldown. We have used group charts for this, at each level we have 2 different group charts one that shows the average of field1 and other the average of field2, the groups defined for this report are Zone and assert. Average is to be calculated at zone level

The catch is that the average calculation is different for this 2 fields.

For field1 the average is our simple average i.e sum of field2 value for each assert under the zone divided by no. of assert under the zone.

For field2 to calculate the average we cannot consider all the assert under the zone, we need to consider only those asserts for which another field say field3 is not null.

Is it possible to achieve this. Can we modify the summary field used in group chart or can we create any formula to calculate the average and use it in group chart.