Returns or sets the number of periods (or units on a scatter chart) that the trendline extends backward. Read/write Long
Example
This example sets the number of units that the trendline on Chart1 extends forward and backward. The example should be run on a 2-D column chart that contains a single series with a trendline.
With Charts("Chart1").SeriesCollection(1).Trendlines(1)
.Forward = 5
.Backward = .5
End With