code: function (threshold) {
  		var axis = this,
  			isLog = axis.isLog;
  
  		var realMin = isLog ? lin2log(axis.min) : axis.min,
  			realMax = isLog ? lin2log(axis.max) : axis.max;
  		
  		if (realMin > threshold || threshold === null) {
  			threshold = realMin;
  		} else if (realMax < threshold) {
  			threshold = realMax;
  		}
  
  		return axis.translate(threshold, 0, 1, 0, 1);
  	}
docs: Get the zero plane either based on zero or on the min or max value.
  Used in bar and area plots