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

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')

Just add to the top answer, if you want do the same thing with figures in cols.

g = sns.FacetGrid(df_long, col="variable", size=5, aspect=1.5,col_wrap=1,sharey=False)
# df_long is a long table with 3 variables
g.map(sns.boxplot, "label", "value", palette='Set2')
g.axes[0].axhline(1, ls='--',c='r')
g.axes[1].axhline(1, ls='--',c='r')
g.axes[2].axhline(0.5, ls='--',c='r')
g.map(plt.xticks, rotation=70) 
plt.show()

result