A dot in column x, counting from left, and row y, counting from the top,
means that key number x has signed key number y.
A red dot means a signature of level 0 or 1, a blue dot level 2 and a green level 3.
Signatures which are not on the primary User ID are represented by a darker dot.
This image has been scaled 41 times from 24190x24190 to 590x590. Each row and each column in the image represents 41 actual OpenPGP keys. A gamma value of 15 was used to keep the image from being too dark to be visible.
Wherever you hold your mouse pointer, you might see in the status bar the MSD and (spam protected) name of the key that has made the signature you are pointing at. Or you might not. It depends on your browser. This web page has a client side image map with key information masking as URLs. It works for me in Mozilla based browsers but your mileage may vary. No ECMAScript was armed during the production of this web page.
Too see the signed key instead of signing key, click here. Since this image is scaled, you cannot see all keys this way. Only the key being closest to the middle of the column is shown.