Plot specific element values in matplotlib

Question

I have a list as below:

freq = [29342, 28360, 26029, 21418, 20771, 18372, 18239, 18070, 17261, 17102]

I want to show the values of n-th and m-th element of the x-axis and draw a vertical line

plt.plot(freq[0:1000])

For example in the graph above, the 100th elements on the x-axis - how can I show the values on the line?

I tried to knee but it shows only one elbow. I suggest it is the 50th element? But what is exactly x,y??

from kneed import KneeLocator
kn = KneeLocator(list(range(0, 1000)), freq[0:1000], curve='convex', direction='decreasing')
import matplotlib.pyplot as plt
kn.plot_knee()
#plt.axvline(x=50, color='black', linewidth=2, alpha=.7) 
plt.annotate(freq[50], xy=(50, freq[50]), size=10)

That '100' is fixed? If you want to show a vertical line you can use plt.axvline(x=100, color='black', linewidth=2, alpha=.7) plt.annotate(freq[100], xy=(100, freq[100]), size=8) — solopiu
– solopiu, Commented Nov 24, 2020 at 15:32
@solopiu Or define your own annotation function that takes ax, a list of x-indexes, and freq as parameters and annotates your curve as you just have shown. Surprisingly, I did not find easily a duplicate, so go ahead - I will upvote it. — Mr. T
– Mr. T, Commented Nov 24, 2020 at 16:01
Thanks a lot, it works, Actually I want to apply zipf distribution of my corpus and find elbow points values. I draw first elbow point with kneelocator but ı did not found the value of elbow. — babeyh
– babeyh, Commented Nov 24, 2020 at 16:23
kn.plot_knee() shows an elbow, but what is the value of it? I think if I merge two plot, I can guess the values x and y on the elbow exactly. — babeyh
– babeyh, Commented Nov 24, 2020 at 16:33

Mr. T · Accepted Answer · 2020-11-24 17:35:09Z

3

You might think that everybody knows this library kneed. Well, I don't know about others but I have never seen that one before (it does not even have a tag here on SO).
But their documentation is excellent (qhull take note!). So, you could do something like this:

#fake data generation
import numpy as np
x=np.linspace(1, 10, 100)
freq=x**(-1.9) 

#here happens the actual plotting
from kneed import KneeLocator
import matplotlib.pyplot as plt

kn = KneeLocator(x, freq, curve='convex', direction='decreasing')
xk = kn.knee
yk = kn.knee_y

kn.plot_knee()

plt.annotate(f'Found knee at x={xk:.2f}, y={yk:.2f}', xy=(xk*1.1, yk*1.1) )

plt.show()

Sample output:

edited Nov 24, 2020 at 17:35

answered Nov 24, 2020 at 17:24

Mr. T

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