在用plt.subplots畫多個子圖中，ax = ax.flatten()將ax由n*m的Axes組展平成1*nm的Axes組

以下面的例子說明ax = ax.flatten()的作用：

fig, ax = plt.subplots(nrows=2,ncols=2,sharex='all',sharey='all')
ax = ax.flatten()  

for i in range(4):
    img = image[i].reshape(28, 28)
    ax[i].imshow(img, cmap='Greys', interpolation='nearest')  # 區別：可以直接用ax[i]
 

不使用ax = ax.flatten()

fig, ax = plt.subplots(nrows=2,ncols=2,sharex='all',sharey='all') 

for i in range(4):
    img = image[i].reshape(28, 28)

    axs[0, 0].imshow(img, cmap='Greys', interpolation='nearest')  # 區別：不能直接使用ax[i]

    axs[0, 1].imshow(img, cmap='Greys', interpolation='nearest')

    axs[1, 0].imshow(img, cmap='Greys', interpolation='nearest')

    axs[1, 1].imshow(img, cmap='Greys', interpolation='nearest')