Highlight/shade points in a scatter plot of a semi-transparent patch

I have a scatter point with datapoints from different categories. I would like to highlight each category with a semi-transparent patch, as a "guide for the eye". Until now, I was drawing an ellipse to achieve this. This works only partly good, especially if the data points lie on a bent curve.

Also, an automated fashion to "cluster" those points would be nice.

Attached you see a version with ellipses, as it is right now... Especially the yellow part would be nice to be "banana-like" bent

[Adam]

What is your actual question? An automated clustering algorithm that considers all those yellow points to be in the same group, and comes up with some kind of banana-shaped group to define that would not be trivial. But you said the data come from categories so are they not automatically clustered anyway, as in your plot above?

[Tom DeLonge]

Sorry for not being clear. The data is clustered. I just want to highlight each cluster. The "algorithm" should just draw a smooth patch around that cluster.