Among the revelations:
- Paul tried using transparent lines (using the alpha option to the rgb function), but there was so much data that he got either white blobs, or nothing.
- The "semitransparent" lines were actually created with a color ramp (via the colorRampPalette function) from black to blue to write, according to friend density.
- The order in which the lines are drawn (darker first, brighter last) is crucial to achieving the desired affect. (Ordering elements in an R chart is an often-overlooked part of creating an effective visual display.)
- Paul wrote his own code to draw the Great Circle arcs (the geosphere package provides similar functionality)
- Only the Facebook logo and the date were added in post-production; the rest of the chart was created entirely in R.
Read the complete details in Paul's post linked below.