By using the number of murders in large cities (over 250k) combined with population, we can determine the murder rate in those cities.
Updated missing url https://cdn.rawgit.com/Caged/d3-tip/v0.8.0-alpha.1/index.js to https://cdn.jsdelivr.net/gh/caged/d3-tip/v0.8.0-alpha.1/index.js
https://d3js.org/d3.v4.min.js
https://d3js.org/topojson.v1.min.js
https://cdn.rawgit.com/Caged/d3-tip/v0.8.0-alpha.1/index.js