Peter,

Using the Border property of SeriesCollection(i)

The object browser shows Border is a property of SeriesLines which is a
property of ChartGroup. the help system says:

With Charts("Chart1").ChartGroups(1)
.HasSeriesLines = True
With .SeriesLines.Border
.LineStyle = xlThin
.Weight = xlMedium
.ColorIndex = 3
End With
End With

I was able to change the series line formatting with
SeriesCollection(i).Border

Arne