lib/utils.js
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);
}
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);
}
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 ) ; }
Calculates area of a polygon.  Ported from d3.js
@param poly Object
@returns {number} square units inside the polygon
Calculates area of a polygon . Ported from d3 . js