I have uploaded a slightly improved version now, to the same link. This should make cycling through coinciding dots possible, (as long as not more than 5 coincide). I do this by suppressing clicked-but-rejected ads, by lifting them 'out of the plane', as it were. (Which makes their apparent distance to any click larger.) Each click on another ad makes them 'fall back' towards the plane, so that the most-recently viewed ad is always most distant, and never clicked or clicked longest ago are closest (and thus selected). This mechanism should also be helpful for dots that do not exactly coincide, biasing the choice towards those not recently clicked.
This version also offsets dots horizontally a little, dependant on their color, so that unequal colors never will completely cover each other, and will remain visible.
I notced that Dasher uses shape to distiguish computer / human (square / cdot), color to indicate the variant (green = normal, magenta = crazyhouse, orange = losers, red = loadable), while open / closed symbols are used to indicate unrated / rated. Parhaps I should copy this in a future version.
[edit] I now uploaded a WinBoard version that spontaneously shows the seek-ad text when you hover the mouse pointer over it.