For future reference.

You can't set the properties of the markers until AFTER the comObject for the series and the markers hve been created, so It seems that the best way to do this that I've found is to (re) set the piMarkerSize and peMarkerType properties in the "loaddata" method(s)

Code:
 
                        Procedure OnAddChartData                             Integer iBytes
                            Set piMarkerSize to 1
                            Get GetDataFromPLC of oGraphDataLoader to iBytes 
                            If (ibytes > 0) Begin 
                                Send AddPoints (ptData(oGraphDataLoader))  
                            End
                        End_Procedure