Here's the key:

Blue pins represent the 1960s (plain for William Hartnell, dotted for Patrick Troughton).

Green pins represent the 1970s (plain for Jon Pertwee, dotted for Tom Baker).

Yellow pins represent the 1980s (plain for Peter Davison, dotted for Colin Baker, pins for Sylvester McCoy).

Red pins represent the contemporary series (plain for Christopher Eccleston, dotted for David Tennant and pins for Matt Smith).