Forked from d3 legend, updated to work with d3 v4.
d3.legend is a quick hack to add a legend to a d3 chart. Simply add a g and .call(d3.legend). Any elements that have a title set in the "data-legend" attribute will be included when d3.legend is called. Each title will appear only once (even when multiple items define the same data-legend) as the process uses a set based on a existing names, not an array of all items.
By default the color in the legend will try to match the fill attribute or the stroke attribute of the relevant items. Color can be explicitly defined by attribute "data-legend-color"