docs: function Gauge (el, opts) {
    if (!(this instanceof Gauge)) return new Gauge(el, opts);
  
    this._el = el;
  
    this._opts = xtend(defaultOpts, opts);  
  
    this._size   =  this._opts.size;
    this._radius =  this._size * 0.9 / 2;
  
    this._cx     =  this._size / 2;
    this._cy     =  this._cx;
  
    this._preserveAspectRatio = this._opts.preserveAspectRatio;
  
    this._min    =  this._opts.min;
    this._max    =  this._opts.max;
    this._range  =  this._max - this._min;
  
    this._majorTicks = this._opts.majorTicks;
    this._minorTicks = this._opts.minorTicks;
  
    this._needleWidthRatio = this._opts.needleWidthRatio;
    this._needleContainerRadiusRatio = this._opts.needleContainerRadiusRatio;
    
    this._transitionDuration = this._opts.transitionDuration;
    this._label = this._opts.label;
  
    this._zones = this._opts.zones || [];
  
    this._clazz = opts.clazz;
  
    this._initZones();
    this._render();
  }