Assign variable to graph column in excel vba

I want change the columns AN, Z, AD from text to the column number which I will store as a integer variable.

            AChart.SeriesCollection.NewSeries
            AChart.FullSeriesCollection(iCollection).Name = "=" & "'" & ws.Name & "'" & "!$AN$" & iStart
            AChart.FullSeriesCollection(iCollection).XValues = "=" & "'" & ws.Name & "'" & "!$Z$" & iStart & ":$Z$" & iEnd
            AChart.FullSeriesCollection(iCollection).Values = "=" & "'" & ws.Name & "'" & "!$AD$" & iStart & ":$AD$" & iEnd

I tried "!C" & variable name but that didn't work.

Please help

Thanks

Dan