[R] How to interpret and modify "plot.svm"?

[Frank Duan]

Dear R people,

I am trying to plot the results from running svm in library(e1071). I
use plot.svm. After searching through the help archives and FAQ, I
still have several questions:

1.  In default, crosses indicate support vectors. But why are there
two colors of crosses? What do they represent?

2. I want to draw a white-gray colored plot and modify the different
colored crosses or circles by different shaped points. Could anyone
give me a hint?

3. Is it possible for me to draw a "hyperplane" on the plot?

4. What is the algorithm to plot the contour region?

Thank you very much,

Frank