Then I stumbled upon SpreadsheetLight which also has good docs and does support some charting. I also like the author's attitude and his commitment to his product. However, the project is built on top of the OpenXML 2.0 SDK, not 2.5, which is a deal breaker for me because I want to have the flexibility to work directly with OpenXML in the event that the library doesn't quite meet my needs. My project is targeting excel 2013 so I want to have the latest SDK to work with....Update! See ErrCode's comment below for additional info...
private void EnableMultiLevelLabels(ExcelChart chart)
{
var element = chart.ChartXml.GetElementsByTagName("c:catAx")[0];
if (element == null)
return;
var multiLevelLabelNode = element.AppendChildElementNode("c:noMultiLvlLbl");
if (multiLevelLabelNode != null)
multiLevelLabelNode.AppendNodeAttribute("val", "0");
}