docs: def bestfit(self):
  	"""
  	Returns a series with the bestfit values. 
  	
  	Example:
  		Series.bestfit()
  
  	Returns: series
  		The returned series contains a parameter 
  		called 'formula' which includes the string representation 
  		of the bestfit line. 
  	"""
  	# statsmodel cannot be included on requirements.txt
  	# see https://github.com/scikit-learn/scikit-learn/issues/4164
  	# which shares the same issue as statsmodel
  	try:
  		import statsmodels.api as sm
  	except:
  		raise Exception("statsmodels is required: " \
  						"please run " \
  						"pip install statsmodels" )
  
  	x=pd.Series(list(range(1,len(self)+1)),index=self.index)
  	x=sm.add_constant(x)
  	model=sm.OLS(self,x)
  	fit=model.fit()
  	vals=fit.params.values
  	best_fit=fit.fittedvalues
  	# the below methods have been deprecated in Pandas
  	# model=pd.ols(x=x,y=self,intercept=True)
  	# best_fit=model.y_fitted
  	best_fit.formula='%.2f*x+%.2f' % (vals[0],vals[1])
  	return best_fit