[R] [Mailman] question: contour plot for discrete data

[L Z]

Question:
 I have matrix (n x3) that represents discrete data.
Each row of matrix is 3-D point (x,y,z). I would like
to get contour map (z value) at two dimension
(x,y). How can I use related contour function to do
this job?
I am not sure if I clarify this question. For example,
we can get point (x,y)
at 2 dimension according to first two columns of
matrix. Then I want to connect
same value z=(x,y). Thanks!
  zhang

The data file looks like this:
X          y               z
4           0.33          0.99
4           0.5            1.2
5           0.66          1.2
5           0.7            1.5
6           2               1.2
?