Hi Neil,
I think you may have to re-arrange data using the Combined query with Union as below and once both quarter data and both amount data comes into a single dimension and measure, you can plot 'Quarter' dimension on X axis and then write formula for separating out inflow amount and outflow amount at report level as below. This is what i could think of seeing data you have,hope others come up with better tricks without re-arranging data.
IF amt=[Amount] Where ([Flag]="IQ")
OF amt=[Amount] Where ([Flag]="OQ")
Select
Project,
Outflow quarter As 'Quarter',
'OQ' as Flag
outflow Amount As 'Amount'
from
where
Union
Select
Project,
Inflow quarter As 'Quarter'
'IQ' as Flag
Inflow Amount As 'Amount'
from
where
Thanks
Mallik