Opencv histogram

import cv2
import matplotlib.pyplot as plt
def main():
    path = "D:\\image_processing_files\\Dataset\\"
    imgpath = path + "4.2.07.tiff"
    img = cv2.imread(imgpath,1)
    img = cv2.cvtColor(img,cv2.COLOR_BGR2RGB)
    
    red_hist = cv2.calcHist([img],[0],None,[256],[0,255])
    green_hist = cv2.calcHist([img],[1],None,[256],[0,255])
    blue_hist = cv2.calcHist([img],[2],None,[256],[0,255])
    
    plt.subplot(3,1,1)
    plt.title("Red Histogram")
    plt.plot(red_hist,color = 'r')
    plt.xlim(xmin = 0,xmax = 256)
    
    plt.subplot(3,1,2)
    plt.title("Green Histogram")
    plt.plot(green_hist,color = 'g')
    plt.xlim(xmin = 0,xmax = 256)

    plt.subplot(3,1,3)
    plt.title("BLue Histogram")
    plt.plot(blue_hist,color = 'b')
    plt.xlim(xmin = 0,xmax = 256)
    plt.show()
    
if __name__ == "__main__":
    main()