绘制简单图表触发ValueError错误，寻求解决方案（附代码）

Hey there! That error is popping up because your function y(t) isn't returning any value—let's sort that out quickly.

The Root Cause #

Your function definition is missing a return statement. When you call y(time), since there's no explicit return, it defaults to sending back None instead of the array of cosine values you calculated. Matplotlib's plot() can't handle a None for the y-axis data, which is exactly why you're seeing that VALUE ERROR: x, y, and format string must not be None.

Corrected Code #

Here's the fixed version of your code:

import numpy as np
import matplotlib.pyplot as plt
%matplotlib inline

time = np.linspace(0, 2, 500)
pi = np.pi

def y(t):
    return np.cos((2 * pi * 10 * t) + 30)  # Added return statement here

plt.plot(time, y(time))
plt.show()  # Optional but explicit way to render the plot

What Changed? #

• Added the return keyword in the y(t) function so it actually outputs the computed cosine values instead of None.
• Added plt.show() to ensure the plot renders clearly (while %matplotlib inline handles this in Jupyter environments, it's still good practice to include for consistency).

Run this code, and you should see your cosine wave plot without any errors!

内容的提问来源于stack exchange，提问作者eoinc837