In the dialog where you set up the group (Dimensions tab --> Edit groups... --> select the group and press Edit) you can set how every field has to sort (Sort command... button)
If you need a different sort for this graph you can use getcurrentfield funtion to catch the current field in the group.
Hope this helps.
Thanks for the reaction but i do not think that this is what i need.
When year is shown, i want to sort by X year Ascending, but when drilling down to X customer, i want to sort by Y-value eg the totals per customer.
So what i need to do in the sort tab expression is if(getcurrentfield(drillDownYearCustomer='Year', *sort by year ascending*,*sort by Y-value descending*) But i do not know how to code the parts between the **
Ok i got it now
to solve the problem i needed to check "expression" under the sort tab and enter
if(getcurrentfield(drillDownYearCustomer)='year', 'year', sum(amount)) with descending
Next i checked "Numeric value" with ascending
Meaning years would be ascending and total amounts would be descending