code: function yPos(dataSet, iteration, add,value) {
  		value = value ? 1*data.datasets[dataSet].data[iteration] : 0;
  		return xAxisPosY - calculateOffset(config.logarithmic, value+add, calculatedScale, scaleHop);
  	}
docs: Get a y position depending on the current values
  @param dataset {integer} number of dataset
  @param iteration {integer} number of value inside dataset.data
  @param add {float} add a value to the current value if value is true
  @param value {bool} true => value+add, false=>add
  @returns {float} position (px)