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None of the analytics packages commonly mentioned here can do cluster analysis, that I know of. SAS and SPSS are the main programs I know of that can do it for web sites. To get into this kind of thing you need many thousands $$$.
And, for certain ordinary stats packages (WebTrends and Omniture for sure, not definite about others) it is possible for a statistically knowledgeable person to take certain output from those packages and, with help from Excel and a cheap stats package, do a site-content type of cluster analysis with a fair amount of work. The visitor type of cluster analysis would then be a few more labor-intensive steps.
Cluster analysis is finding groups of visitors based on specific attributes. For example, by using SAS I can find a cluster of visitors based on similar page views per session distributions. SAS will dictate the clusters.
Segmentation analysis, on the other hand, takes a more simple approach. You have to manually set up your segment. For example Segment A are visitors with less than two page views per session, segment B are visitors with between two and five page views per session, and segment C are visitors with greater than 5 page views per session.
Hope this helps.
Or are you referring to a scatter plot? What are the axes and bubble size variable in your graph? If you are managing to do a real cluster analysis I'd love to know how.