Posts are dispersed on the time axis, left to right, and a circle is drawn connecting posts which share tags. The circle is drawn with a lower alpha value, so darker circles mean that more than one tag is shared. It's not that meaningful, save from showing that I use more tags per post than when I started using del.icio.us (nothing a quick graph wouldn't show too), but it's a pleasing pattern never the less.
Plimpplampplettere, by the way, is almost certainly made up, and not really the Dutch word for "skimming stones" (Thanks to Ron for clarification).