Hi anatolia,
The chart is produced by a macro in the exceldem.xls spreadsheet file in the example's directory. You could change the plot of the macro, or make a new one to chart a line.
Here is a KnowledgeBase with some helpful links.
Using National Instruments Software to Control Microsoft Excel with ActiveX
http://digital.ni.com/public.nsf/websearch/56308664EB597E9E8625696500631D92?OpenDocument
Gavin Fox
Systems Software
National Instruments