R help - Mar 2008 - How to plot raw data and fit curve on the same figure and how to decide how good the fitting is?

Dear R users,
I have two group of data (say X1 and X2) that (may) fit bivariate
normal distribution.
I used mvnxxx (a function in mclust library) to fit the data to a
bivariate normal
distribution.

Now I want to plot the data (each (X1, y1) represents a point on the
xy plane) and
the estimated bivariate normal distribution curve on the same figure to see how
good the fit line is. How can I plot them on the same figure?

Besides plot them on the same figure, is there any more "systematic"
way to check
how good the fitness is?

Thanks a lot in advance.