A perceptually accurate pie chart.
inspired by this photo, whose provenance has been lost in the sands of time
the legend is drawn with the handy d3-legend component from @DataToViz
a descendant of this bl.ock from the prolific currankelleher
forked from micahstubbs's block: Pyramid Pie
https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js
https://cdnjs.cloudflare.com/ajax/libs/d3-legend/1.1.0/d3-legend.js
https://npmcdn.com/babel-core@5.8.34/browser.min.js