Currently it is not possible to add null-able series items in RadHtmlChart in the markup or programmatically. If a null-able item is added it is omitted and is replaced by the next series item from the current Series. The workaround is to either data bound the chart to a data source or convert the null-able values to zeros: C#: decimal? nullValue = null; if (nullValue == null) { nullValue = 0; } ColumnSeries colSeries2 = RadHtmlChart1.PlotArea.Series[1] as ColumnSeries; colSeries2.SeriesItems.Add(39); colSeries2.SeriesItems.Add(nullValue); colSeries2.SeriesItems.Add(3);