That's a dot for each of the 2,000 most commonly-used color names as harvested from the 5,000,000-plus-sample results of XKCD's color survey, sized by relative usage and positioned side-to-side by average hue and vertically by gender preference. Women tend to use color names nearer the top, men towards the bottom, and the dashed line represents the 50-50 split (equal usage by both sexes).
While his original version was static, the interactive version lets you sort by hue, saturation, brightness, popularity, and name length. Most importantly, you can see the color names now when you mouse over. I like the vertical spectrum of purple, where women use names like bright lilac, orchid, and heather, and men tend to label similar shades as purplish, lightish purple, and oh yes, very light purple. [Thanks, Stephen]