code: def addField(self, field) :
  		
  		if field.lower() in self.legend :
  			raise  ValueError("%s is already in the legend" % field.lower())
  		self.legend[field.lower()] = len(self.legend)
  		if len(self.strLegend) > 0 :
  			self.strLegend += self.separator + field
  		else :
  			self.strLegend += field
docs: add a filed to the legend