Seaborn: how to draw a vertical line that matches a specific y value in a cumulative KDE?

Question

I'm using Seaborn to plot a cumulative distribution and it's KDE using this code:

sns.distplot(values, bins=20, 
             hist_kws= {'cumulative': True}, 
             kde_kws= {'cumulative': True} )

This gives me the following chart:

I'd like to plot a vertical line and the corresponding x index where y is 0.8. Something like:

How do I get the x value of a specific y?

Answer 1

Update for newer Seaborn versions, with sns.histplot

The code below has been tested with Seaborn 0.13.2. The 0.8 quantile is calculated and shown as an annotated vertical line.

import matplotlib.pyplot as plt
import seaborn as sns
import numpy as np

values = np.random.normal(1, 20, 1000)
ax = sns.histplot(values, bins=20, cumulative=True, kde=True, color='salmon')
perc = 80
x80 = np.quantile(values, perc / 100)
ax.axvline(x80, color='b')
ax.text(x80, 0.98, f"{perc}th percentile: \nx={x80:.2f} ", color='b',
        ha='right', va='top', transform=ax.get_xaxis_transform())
ax.margins(x=0)
plt.show()

Old answer

You could draw a vertical line at the 80% quantile:

import matplotlib.pyplot as plt
import numpy as np
import seaborn as sns

values = np.random.normal(1, 20, 1000)
sns.distplot(values, bins=20,
             hist_kws= {'cumulative': True},
             kde_kws= {'cumulative': True} )
plt.axvline(np.quantile(values, 0.8), color='r')
plt.show()

Answer 2

The answer by @JohanC is probably the best. I went an other route and it's maybe a slightly more generic solution.

The idea is to get the coordinates of the kde line, then find the indice of the point where it crosses the threshold value

values = np.random.normal(size=(100,))
fig = plt.figure()
ax = sns.distplot(values, bins=20, 
             hist_kws= {'cumulative': True}, 
             kde_kws= {'cumulative': True} )

x,y = ax.lines[0].get_data()
thresh = 0.8
idx = np.where(np.diff(np.sign(y-thresh)))[0]
x_val = x[idx[0]]
ax.axvline(x_val, color='red')