What's the easiest way to separate two populations in a scatterplot?

I have to separate two populations by a line in a scatterplot:

I would like find a threshold that separates the two populations. In @Waynes words, I would like to cluster the points into two categories, then calculate the line which best separates them. What is the easiest way to accomplish this in R? I've tried the Otsu algorithm on the raw data, but I don't know if this one is robust enough for other datasets.

Edit: I don't know which dot belongs to which population. The data is not multivariate, I only have an X and a Y value, as seen in the dotplot.

• Do you know which dots belong to which population? (Ie, is this cluster analysis?) Are you wondering how to get the line, or how to plot a line that you already have? – gung - Reinstate Monica Mar 4 '14 at 21:10
• No, I don't know which dot belongs to which population. The data is not multivariate, I only have an X and a Y value, as seen in the dotplot. I want to get the line. – Eekhoorn Mar 4 '14 at 21:17
• So you're asking how to cluster the points into two categories, then calculate the line which best separates them? – Wayne Mar 4 '14 at 21:19
• @gung For the sake of easiness it should be straight. But if you have other approaches leading to a curved line, I'm also interested for future applications. – Eekhoorn Mar 4 '14 at 21:21
• @gung I have the actual values. – Eekhoorn Mar 4 '14 at 21:23