Trendlines Method
Returns an object that represents a single trendline (a Trendline object) or a collection of all the trendlines (a Trendlines collection) for the series.
object.Trendlines(Index)
object Required. The Series object.
Index Optional Variant. The name or number of the trendline.
Example
This example adds a linear trendline to series one in Chart1.
Charts("Chart1").SeriesCollection(1).Trendlines.Add Type:=xlLinear