def addField(self, field) :
		"""add a filed to the legend"""
		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