Finish times vs. number of runners in the Bostom Marathon, year over year. Darker means that more people ran times in that hour range, which is specified in the y axis.
This is related to the buzz about slow runners in marathons. There is definitely an expansion in the >4hr set as the data goes on. The limited results in 2013 are, of course, because of the Boston Marathon bombings.
Modified http://d3js.org/d3.v3.min.js to a secure url
https://d3js.org/d3.v3.min.js