Draw a line at specific position/annotate a Facetgrid in seaborn

A have produced a boxplot with Facetgrid in seaborn the following way

# Import the dataset
tips = sns.load_dataset("tips")

# Plot using FacetGrid, separated by smoke
plt.style.use('ggplot')
g = sns.FacetGrid(tips, col="smoker", size=5, aspect=1.5)
g.map(sns.boxplot, "sex", "total_bill", palette='viridis', order=['Male', 'Female'])
plt.show()

I now want to draw to distinct horizontal lines in each of the plot. For instance one horizontal line (with coordinated (0,10)) only in the left-hand side plot and another horizontal line (with coordinates (0,30)) only in the right-hand side plot.

How can I go about doing that?

You can get a list of axes used in the FacetGrid using FacetGrid.axes which returns the axes used. You can then do all of the normal matplotlib operations using these axes, such as axhline for horizontal lines, or plt.text for putting text on the axes:

import seaborn as sns
import matplotlib.pyplot as plt
tips = sns.load_dataset("tips")

# Plot using Facegrid, separated by smoke
plt.style.use('ggplot')
g = sns.FacetGrid(tips, col="smoker", size=5, aspect=1.5)
g.map(sns.boxplot, "sex", "total_bill", palette='viridis', order=['Male', 'Female'])

ax1, ax2 = g.axes[0]

ax1.axhline(10, ls='--')
ax2.axhline(30, ls='--')

ax1.text(0.5,25, "Some text")
ax2.text(0.5,25, "Some text")

plt.show()

Additionally, if you have a bunch of grids that you want to add one horizontal line (say at y=10) to all then you can just "map" the "plt.axhline" with your grid object:

import seaborn as sns
import matplotlib.pyplot as plt
tips = sns.load_dataset("tips")
# Plot using Facegrid, separated by smoke
plt.style.use('ggplot')
g = sns.FacetGrid(tips, col="smoker", size=5, aspect=1.5)
g.map(sns.boxplot, "sex", "total_bill", palette='viridis', order=['Male', 'Female'])

g.map(plt.axhline, y=10, ls='--', c='red')