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

x1返回的是x1min到x1max间均匀分成num1个数，进行横向扩展为方阵

x2返回的是x2min到x2max间均匀分成num2个数，进行纵向扩展为方阵

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. ])