def surface(n_x=20,n_y=20):
	"""
	Returns a DataFrame with the required format for 
	a surface plot

	Parameters:
	-----------
		n_x : int
			Number of points along the X axis
		n_y : int
			Number of points along the Y axis
	"""	
	x=[float(np.random.randint(0,100))]
	for i in range(n_x):
		x.append(x[:1][0]+np.random.randn()*np.random.randint(1,10))
	df=pd.DataFrame(x)
	for i in range(n_y):
		df[i+1]=df[i].map(lambda x:x+np.random.randn()*np.random.randint(1,10))
	return df