Events are drawn as dots but these are expanded with gaussian blur and then thresholded into visual blobs where height suggests number.
Event Drops: A time based / event series interactive visualization using d3.js with gaussian blur to aggregate events into visual blobs called metaballs. site 
Metaballs are, in computer graphics, organic-looking n-dimensional objects. The technique for rendering metaballs was invented by Jim Blinn in the early 1980s. wikipedia
The effect is coded as a three stage filter, Gaussian Blur ⇒ Color Matrix ⇒ Blend. github