解决：UserWarning: Matplotlib is currently using agg, which is a non-GUI backend

import matplotlib
print(matplotlib.get_backend())

#matplotlib.use('TkAgg')

import matplotlib.pyplot as plt
print(matplotlib.get_backend())


import numpy as np
print(matplotlib.get_backend())

import pylab
print(matplotlib.get_backend())

matplotlib.use('QT5Agg')
print(matplotlib.get_backend())

import objtracker
from objdetector import Detector
import cv2

import pandas as pd

matplotlib.use('QT5Agg')
print(matplotlib.get_backend())

大体方法，在每个import之后添加print(matplotlib.get_backend())语句，查看是哪个语句使得matplotlib.get_backend()发生了变化。我这里是import了pandas后变为了agg，所以在import pandas as pd后面紧跟了一个matplotlib.use('QT5Agg')，就可以正常画图了。