[AccessD] Graph Rowsource

AccessXP ADP,  SQL2000

I am trying to change a graphs rowsource via VBA.   When I have the 
following in the Detail_Format section of the report -

     Me!chtincNeeds.RowSource = "SELECT C1stName ...  " (Rest of query omitted)

I get an error 2191 - You can't set the rowsource property in print preview 
or after printing has started.

If I put it on the report's on open event I get error 2455 - You entered an 
expression that has an invalid reference to the property RowSource.

How can I dinamically change the rowsource?