Phan,
Please see the Waveform Graph.vi example in \examples\general\graphs\gengraph.llb. This example shows many different ways of plotting multiple plots on to a graph.
As a general answer, to plot multiple plots on a chart, create a bundle where each plot is one item in the bundle. To plot multiple waveforms to a graph, build an array of waveforms where each array item is one plot.
Randy Hoskin
Applications Engineer
National Instruments
http://www.ni.com/ask