x1,x2 = np.mgrid(x1min:x1max:num1j,x2min:x2max:num2j)

x1返回的是x1min到x1max間均勻分成num1個數，進行橫向擴展為方陣

x2返回的是x2min到x2max間均勻分成num2個數，進行縱向擴展為方陣

Examples
--------
>>> np.mgrid[0:5,0:5]
array([[[0, 0, 0, 0, 0],
??????? [1, 1, 1, 1, 1],
??????? [2, 2, 2, 2, 2],
??????? [3, 3, 3, 3, 3],
??????? [4, 4, 4, 4, 4]],
?????? [[0, 1, 2, 3, 4],
??????? [0, 1, 2, 3, 4],
??????? [0, 1, 2, 3, 4],
??????? [0, 1, 2, 3, 4],
??????? [0, 1, 2, 3, 4]]])
>>> np.mgrid[-1:1:5j]
array([-1. , -0.5,? 0. ,? 0.5,? 1. ])