matplotlib 绘制一个灰度直方图

import cv2 as cv
import matplotlib.pyplot as plt
img = cv.imread("./lena.jpg")
img_gray = cv.cvtColor(img, cv.COLOR_RGB2GRAY)
grayDict = {}
for key in range(256):
    grayDict[key] = 0
for i in range(img_gray.shape[0]):
    for j in range(img_gray.shape[1]):
        grayDict[img_gray[i][j]] += 1
plt.figure(dpi=300)
plt.xticks(range(0,255,25))
plt.bar(list(grayDict.keys()), list(grayDict.values()))
plt.savefig("hist.png")
plt.show()