code: function _polarRadian(xyCenter, xyPointsArray) {
    // simplify the math by moving the computed circle center to [0, 0]
    var xAdj = 0 - xyCenter[0],
        yAdj = 0 - xyCenter[1];
  
    return _.map(xyPointsArray, function(xy){
      // reposition x and y relative to adjusted [0, 0] center
      var x = xy[0] + xAdj;
      var y = xy[1] + yAdj;
  
      var adjArcTan = Math.atan2(y, x);
  
      // compensate for quad3 and quad4 results
      if(adjArcTan < 0){
        adjArcTan = 2 * Math.PI + adjArcTan;
      }
  
      return adjArcTan;
    });
  }
docs: _polarRadian compute radian component of polar coordinate relative to a center point
  @param  {Array} xyCenter         [x, y] center point to anchor radial sweep
  @param  {Array} xyPointsArray    Array of [x, y] arrays -- one for each point to index
  @return {Array}                  the radian position (between 0 and 2PI) of each coordinate