Each dot is one runner at the 2014 Boston Marathon, placed by how much slower they are running than their own pace from 5 to 20km. Dots turn red once a runner crosses into the wall.
Source: Boston Athletic Association results, 2014, via github.com/llimllib/bostonmarathon. Random sample of 3,000 men and 3,000 women from the 31,548 finishers with full splits. Wall: running at least 25% slower than your own 5 to 20km pace.
Try this: press play, then pick "3:30 to 4:00". Runners in that group finished in the same time, so the gap can't be explained by men simply being faster.
Pace between split points is interpolated, so motion between checkpoints is smoothed. One race and one course: Boston's hills after 25km make everyone fade more than a flat course would.