Steve,

I've encountered the very problem and - even though the docs state that it should work using ConstantY2 - after skimming the source code I believe that this is not implemented at the moment. Anyway, you can easily work this around by using the Points2 collection

areaSeries.Points.Add(DateTimeAxis.CreateDataPoint(this.time, percentage));
areaSeries.Points2.Add(DateTimeAxis.CreateDataPoint(this.time, 0));

This ain't really beautiful, but it works.