A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
I'm prettty sure you cannot change the default setting, but the following code changes all the series on the active chart to line thickness of 1.5.
Sub FormatAllSeries()
Dim SeriesNum As Series
With ActiveChart
For Each SeriesNum In.SeriesCollection
SeriesNum.Format.Line.Weight = 1.5
Next
End With
End Sub