docs: 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;
    });
  }