Returns a DataFrame with the required format for 
	a distribution plot (distplot)

	Parameters:
	-----------
		n : int
			Number of points 
		categories : bool or int
			If True, then a column with categories is added
		n_categories : int
			Number of categories

def violin(n=500,dispersion=3,categories=True,n_categories=5):
	"""
	Returns a DataFrame with the required format for 
	a distribution plot (distplot)

	Parameters:
	-----------
		n : int
			Number of points 
		categories : bool or int
			If True, then a column with categories is added
		n_categories : int
			Number of categories
	"""	
	df = histogram(1,n,dispersion,'abc')
	df=df.rename(columns={'a':'data'})
	if categories:
		df['categories']=['category_{0}'.format(np.random.randint(n_categories)) for _ in range(n)]
	return df