code: function area(poly) {
      var i = -1,
          n = poly.length,
          a,
          b = poly[n - 1],
          area = 0;
  
      while (++i < n) {
          a = b;
          b = poly[i];
          area += a.y * b.x - a.x * b.y;
      }
  
      return Math.abs(area * 0.5);
  }
docs: Calculates area of a polygon.  Ported from d3.js
  @param poly Object
  @returns {number} square units inside the polygon