Returns a DataFrame with the required format for 
	a heatmap plot

	Parameters:
	-----------
		n_x : int
			Number of x categories
		n_y : int
			Number of y categories

def heatmap(n_x=5,n_y=10):
	"""
	Returns a DataFrame with the required format for 
	a heatmap plot

	Parameters:
	-----------
		n_x : int
			Number of x categories
		n_y : int
			Number of y categories
	"""	
	x=['x_'+str(_) for _ in range(n_x)]
	y=['y_'+str(_) for _ in range(n_y)]
	return pd.DataFrame(surface(n_x-1,n_y-1).values,index=x,columns=y)