Check out sample 9, in the LoadFile2 method, it shows how to do it.
In your case something like this...
var chart = sheet.Drawings.AddChart("chart2", eChartType.Line);
ExcelAddress valueAddress = new ExcelAddress(2, 5, 6, 5);
var ser1=chart.Series.Add(valueAddress.Address, "B2:B6");
//Add a new charttype
var chartType2 = chart.PlotArea.ChartTypes.Add(eChartType.ColumnStacked);
var ser2 = chartType2.Series.Add(valueAddress1.Address, "B2:B6");
(The Chart.Chartype shows the first charttype when the chart is added. It's supposed to be readonly, so it's a bugg. I'll add a fix for that later)