You need to reference the sheet where the cells are
Code:
ActiveChart.SetSourceData Source:=Sheets("Salinity_Obs").Range(Sheets("Salinity_Obs").Cells(Iref, Jref), Sheets("Salinity_Obs").Cells(Iref + Rownum - 1, Jref + Colnum - 1)), _
PlotBy:=xlColumns