plot multi-dataset

Hi, ALL, I tried to use plot command within a for loop to plot

    > multiple set of data. I am surprised to see that the plot for
    > the first dataset also appears on the plot for the second
    > dataset, the plot for the 1st and 2nd dataset appears on the
    > 3rd plot, and so on. Could anyone give me a hand on this issue
    > to erase memory?

See help for

  figure - create new / set current figure
  close - close a specific or all figures
  clf - clear current figure
  cla - clear current axes
  hold - set the "hold" state for current axes
  ishold - return the "hold" state for current axes

In particular, the "hold" setting determines whether subsequent plot
commands overplot previous ones.