Trendlines Method [Excel 2003 VBA Language Reference]
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
Applies to | Series Object