Matplotlib not giving the correct graph of a function

Question

The graph I'm getting of the function v(t) using matplotlib.pyplot as plt and plt.plot(t_test, vel) is not giving me what it is supposed to. Here is the full code and the resulting plot:

import numpy as np
import matplotlib.pyplot as plt

t_test = np.linspace(0,1,200)

def v(t):
    return 1500*(350/2000000) - (350/2000000)*np.log(1-0.00625*t)

vel = v(t_test)

print(vel)
plt.plot(t_test, vel)
plt.show()

The seemingly linear graph is certainly expected since values of t_test are between 0 and 1, so we're only seeing a small portion of the graph. My issue, however, is that the graph is showing v(0) = 0 when in fact, v(0) = 0.2625. Why do I have this discrepancy?

Add `scaley=False` to your `plot` call and you'll get your virtually straight line. — Tim Roberts, Aug 01 '21 at 23:25

score 2 · Accepted Answer · edited Aug 01 '21 at 23:27

2

In fact, it seems to me that you have a correct answer but I think that you didn't realize the y-scale. If you see the top of Y axis, there is 1e-6+2.6250000000e-1, this means that the value that you have on the Y axis you have to multiply by 1e-6 ( i.e. 10^-6) and sum 2.6250000000e-1 ( i.e. 0.2625). So, v(0)=0*(10^-6) + 0.2625 =0.2625.

edited Aug 01 '21 at 23:27

Dharman

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answered Aug 01 '21 at 23:22

Jerick Rodriguez

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I believe you, but why would it do that? How does that make sense as a default action? – Tim Roberts Aug 01 '21 at 23:24
OK, if you add `scaley=False` to the `plot` call, then you will get the result you expect. Nice analysis -- I had not caught this. – Tim Roberts Aug 01 '21 at 23:25
@TimRoberts matplotlib prefers to show an offset text when the calculated tick labels have their first 6 digits equal. – JohanC Aug 01 '21 at 23:31
I'm not pretty sure about why it would do that. But fortunately we all understood why he was getting what he thought was a wrong answer, and you gave him a solution. – Jerick Rodriguez Aug 01 '21 at 23:34
@JohanC that's a good reason. Thanks – Jerick Rodriguez Aug 01 '21 at 23:36

Matplotlib not giving the correct graph of a function

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