Python ValueError:x、y和格式字符串不能为None意味着什么？

以下是我目前的代码：

import statistics
import pandas as pd
import numpy as np
from scipy import stats
import matplotlib.pyplot as plt
from datetime import datetime

start_time = datetime.now()

df = pd.read_csv(r"/Users/aaronhuang/Documents/Desktop/ffp/exfileCLEAN2.csv", skiprows=[1]) 
# replace this with wherever the  file is.

magnitudes = df['Magnitude '].values
times = df['Time '].values
average = statistics.mean(magnitudes)
sd = statistics.stdev(magnitudes)
below = sd*3

i = 0
while(i < len(df['Magnitude '])):
    if(abs(df['Magnitude '][i]) <=  (average - below)):
        tim = print(df['Time '][i])
        mag = df['Magnitude '][i]


    i += 1

window = 2
num = 1
x = tim
y = mag


plt.plot(x, y)
plt.xlabel('Time (units)')
plt.ylabel('Magnitude (units)')
plt.show()

end_time = datetime.now()
print('Duration: {}'.format(end_time - start_time))

目前，它输出以下内容：

/Users/aaronhuang/.conda/envs/EXTTEst/bin/python. 
"/Users/aaronhuang/PycharmProjects/EXTTEst/Code sandbox.py"
2456116.494
2456116.535
2456116.576
2456116.624
2456116.673
2456116.714
2456116.799
2456123.527
2456166.634
2456570.526
2456595.515
2457485.722
2457497.93
2457500.674
2457566.874
2457567.877
Traceback (most recent call last):
  File "/Users/aaronhuang/PycharmProjects/EXTTEst/Code sandbox.py", line 40, in <module>
    plt.plot(x, y)
  File "/Users/aaronhuang/.conda/envs/EXTTEst/lib/python3.8/site- 
packages/matplotlib/pyplot.py", line 2761, in plot
    return gca().plot(
  File "/Users/aaronhuang/.conda/envs/EXTTEst/lib/python3.8/site- 
packages/matplotlib/axes/_axes.py", line 1646, in plot
    lines = [*self._get_lines(*args, data=data, **kwargs)]
  File "/Users/aaronhuang/.conda/envs/EXTTEst/lib/python3.8/site- 
packages/matplotlib/axes/_base.py", line 216, in __call__
    yield from self._plot_args(this, kwargs)
  File "/Users/aaronhuang/.conda/envs/EXTTEst/lib/python3.8/site- 
packages/matplotlib/axes/_base.py", line 322, in _plot_args
    raise ValueError("x, y, and format string must not be None")
ValueError: x, y, and format string must not be None

我不确定如何修复此代码错误。我相信这是由于Tim和mag在该功能中被定义。但我不知道如何解决它。 我希望我的代码能够打印一个带有选定点和10个点的图表。如果有人能帮助解决这个错误，那就太好了。谢谢

变化：

tim = print(df['Time '][i])

致：

tim=printdf['Time'][i]将tim设置为无，因为print不会返回任何内容。这就是错误的原因。替换为tim=df['Time'][i]

print(df['Time '][i])
tim = df['Time '][i]