Sorting in Chart

[neskin]

Hi
how can i sort records in chart
i have month_ref and year_ref fields for ex
month_ref year_ref
1 2008
1 2007
2 2007
3 2007
4 2007
5 2007
6 2007
...
in chart need to dispaly
Jan-07 Feb-07 Mar-07 Apr-07 Jun-07... Jan-08
on change of use i use the formula
stringvar yy;
yy := totext({SPR_LAX_EASTDEPARTURESUMM.YEAR_REF},0);
MonthName({SPR_LAX_EASTDEPARTURESUMM.MONTH_REF},true)+ '-'+ right(yy,2);
then don't know how to sort in order above

thank you