Dia is Open Source software and has many authors. The complete lists of authors is available in Dia's About dialog.
Commits to the Dia repository over time. Note that not all patch contributors are mentioned correctly - in several cases the committers are shown instead.
The video was created with Gource.
Estimating the effort
Ohloh tries to estimate the effort that went into Dia:
The following are the homepages of some of the Dia developers: