In the current version of ExcelWriter, Linear is not a part of the RegressionType Enumeration for charts trendlines.

Therefore it is not possible to use the enumeration in order to create a linear trendline, like the one in the scatter chart below, in one step.

To create a linear trendline:

1) create a Polynomial trendline
2) set the RegressionValue to 1

Trendline trend = oSeries.Trendlines.CreateTrendline(Trendline.Regression.Polynomial);
trend.RegressionValue = 1;

Dim trend As Trendline 
trend = oSeries.Trendlines.CreateTrendline(Trendline.Regression.Polynomial) 
trend.RegressionValue = 1

The linear trendline will be added to the RegressionType enumeration in a future version.