Removing undesired values of a matrix (filtering)

Hi,

I have such a graph.

I want to remove undesired values to get like a following graph.

How can I do that?

[Geoff Hayes]

Why have the ranges of the axes changed? In the first image, the x-axis ranges from 0 to 300+ whereas in the second it is from 350 to 550+. (Same problem for the y-axis too.) What is your method for considering a point undesirable?

[Image Analyst]

Also, can you attach the data file and code to read it in. And let us know if you have the Image Processing Toolbox because there are several functions in there that could possibly help. Why is the dense cluster in the upper left not also one of the desired cluster perimeters?