Add Parabolic SAR (PTPS) study to QuantFigure.studies

		Parameters:
			periods : int or list(int)
				Number of periods
			af : float
				acceleration factor
			initial : 'long' or 'short'
				Iniital position
				default: long
			name : string
				Name given to the study
			str : string
				Label factory for studies
				The following wildcards can be used:
					{name} : Name of the column
					{study} : Name of the study
					{period} : Period used
				Examples:
					'study: {study} - period: {period}'
		kwargs: 
			legendgroup : bool
				If true, all legend items are grouped into a 
				single one
			All formatting values available on iplot()

def add_ptps(self,periods=14,af=0.2,initial='long',str=None,name='',**kwargs):
		"""
		Add Parabolic SAR (PTPS) study to QuantFigure.studies

		Parameters:
			periods : int or list(int)
				Number of periods
			af : float
				acceleration factor
			initial : 'long' or 'short'
				Iniital position
				default: long
			name : string
				Name given to the study
			str : string
				Label factory for studies
				The following wildcards can be used:
					{name} : Name of the column
					{study} : Name of the study
					{period} : Period used
				Examples:
					'study: {study} - period: {period}'
		kwargs: 
			legendgroup : bool
				If true, all legend items are grouped into a 
				single one
			All formatting values available on iplot()
		"""
		study={'kind':'ptps',
			   'name':name,
			   'params':{'periods':periods,'high':self._d['high'],'low':self._d['low'],'af':af,'initial':initial,
						 'str':str},
			  'display':utils.merge_dict({'legendgroup':False},kwargs)}
		self._add_study(study)