How to add droplines to a seaborn scatterplot? - matplotlib

Question

Using the following example code in a Jupyter notebook:
import pandas as pd
import seaborn as sns
import numpy as np
%matplotlib inline
%config InlineBackend.figure_format = 'svg'
df = pd.DataFrame(np.random.rand(5, 2), columns=['a', 'b'])
sns.set()
g = sns.relplot(data=df, x='a', y='b', kind='scatter');
g.set(xlim=(0, 1))
g.set(ylim=(0, 1));
The resulting plot shows the data points, but I would also like to have vertical drop lines and occasionally horizontal ones as well. To clarify what I mean by droplines, here is a mockup of the actual vs. the desired output:
Update: A little more complex input that makes it harder to manually draw the lines:
import pandas as pd
import seaborn as sns
import numpy as np
%matplotlib inline
%config InlineBackend.figure_format = 'svg'
df = pd.DataFrame(np.random.rand(20, 3), columns=['a', 'b', 'c'])
df['d'] = ['apples', 'bananas', 'cherries', 'dates'] * 5
sns.set()
g = sns.relplot(data=df, x='a', y='b', hue='c', col='d', col_wrap=2, kind='scatter');
g.set(xlim=(0, 1))
g.set(ylim=(0, 1));

Answer 1

There are several ways to plot vertical/horizontal lines. One of the is to use hlines or vlines. This can be done using a loop for sake of ease.
import pandas as pd
import seaborn as sns
import matplotlib.pyplot as plt
import numpy as np; np.random.seed(121)
fig, ax = plt.subplots()
df = pd.DataFrame(np.random.rand(5, 2), columns=['a', 'b'])
sns.set()
g = sns.relplot(data=df, x='a', y='b', kind='scatter', color='blue', ax=ax);
for x, y in zip(df['a'], df['b']):
ax.hlines(y, 0, x, color='blue')
ax.vlines(x, 0, y, color='blue')
ax.set_xlim(0, 1)
ax.set_ylim(0, 1)
plt.close(g.fig)