[FIXED] VALUE ERROR: x, y, and format string must not be None. (error while plotting hysteresis loop)

Issue

With the low cycle fatigue data, I'm trying to plot the Hysteresis loop. But I'm getting the following error:

[ -52.18297297  -45.58565338   16.9913185  ... -354.53630032 -295.50857248
-155.42088911]
[-0.01229182 -0.00891753  0.02256744 ... -0.33507242 -0.31283728
-0.24790212]
Traceback (most recent call last):
File "f:\I2M\LCF\Ep1_camp4_P4_TTH650 06-9-21 11 01 24\ep1_camp4_P4.py", line 16, in <module>
plt.plot(strain, Sigma, color = 'k')
File "C:\Users\DELL\AppData\Local\Programs\Python\Python39\lib\site- 
packages\matplotlib\pyplot.py", line 2840, in plot
return gca().plot(
File "C:\Users\DELL\AppData\Local\Programs\Python\Python39\lib\site- 
packages\matplotlib\axes\_axes.py", line 1743, in plot
lines = [*self._get_lines(*args, data=data, **kwargs)]
File "C:\Users\DELL\AppData\Local\Programs\Python\Python39\lib\site- 
packages\matplotlib\axes\_base.py", line 273, in __call__
yield from self._plot_args(this, kwargs)
File "C:\Users\DELL\AppData\Local\Programs\Python\Python39\lib\site- 
packages\matplotlib\axes\_base.py", line 379, in _plot_args
raise ValueError("x, y, and format string must not be None")
ValueError: x, y, and format string must not be None

And here is my code:

import matplotlib.pyplot as plt
import numpy as np
plt.style.use(['science','no-latex'])

x = np.loadtxt('F:\\I2M\\LCF\\Ep1_camp4_P4_TTH650 06-9-21 11 01 24\\data_1.csv',unpack = True, 
skiprows = 2, usecols = 2, delimiter = ',')
y = np.loadtxt('F:\\I2M\\LCF\\Ep1_camp4_P4_TTH650 06-9-21 11 01 24\\data_1.csv',unpack = True, 
skiprows = 2, usecols = 3, delimiter = ',')
stress = (x*1000)/28.27   #N/mm^2 = MPa
length = len(stress)
length = len(y)

plt.figure(figsize=(5, 5))
Sigma = print(stress[0:length:10]) #stress
strain = print(y[0:length:10])


plt.plot(strain, Sigma, color = 'k')

plt.show()

Data contains many rows. So I used some commands to access only particular values from the row

Solution

Your problem is here

Sigma = print(stress[0:length:10]) #stress
strain = print(y[0:length:10])

what you want plausibly is to sample every 10th data point, but what you get is … nothing or, from the point of view of Python: None, so that later your stack trace informs you that x, y, and format string must not be None.

Why this happens, and how you solve the problem?

When you make an assignment, the value of the expression on the right is saved and you can use the name on the left to use it later, so you save, e.g., the value returned by print(y[0:length:10]) to use it later under the name strain, but print() is used for its side effects (i.e., showing a bunch of characters on your terminal) and the value that is returned in these cases is by default None, not what was shown on your terminal.

If I have understood your intentions, you should omit the two lines above and just use

plt.plot(x[0:length:10], y[0:length:10], color='k')

A side note, you have

length = len(stress)
length = len(y)

but you read them from the same file, one assignment should be enough…

---

PS

x, y = np.loadtxt('…\\data_1.csv', unpack=1, skiprows=2, usecols=[2,3], delimiter=',')

Answered By - gboffi