python - Obtaining span of plotted points from seaborn swarmplot

Question

I have the following data:

import pandas as pd
import numpy as np

# Generate dummy data.
a = np.random.random(75)
b = np.random.random(75) - 0.6
c = np.random.random(75) + 0.75 

# Collate into a DataFrame
df = pd.DataFrame({'a': a, 'b': b, 'c': c}) 
df.columns = [list(['WT', 'MUT', 'WTxMUT']), list(['Parent', 'Parent', 'Offspring'])]
df.columns.names = ['Genotype', 'Status']
df_melt = pd.melt(df) 

and I plot it in seaborn using this code:

import seaborn as sb
sb.swarmplot(data = df_melt, x = "Status", y = "value", hue = "Genotype")

How do I get the x-span of each group? What is the range of the horizontal span of the swarmplot for the Parent group, for instance?

Answer 1

You can get the information from the collections which are created by swarmplot.

swarmplot actually returns the matplotlib Axes instance, and from there we can find the PathCollections that it creates. To get the positions, we can use .get_offsets().

Here is your example, modified to find and print the swarm limits, and then use them to plot a box around the swarms.

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
import seaborn as sb
from matplotlib.patches import Rectangle

# Generate dummy data.
a = np.random.random(75)
b = np.random.random(75) - 0.6
c = np.random.random(75) + 0.75 

# Collate into a DataFrame
df = pd.DataFrame({'a': a, 'b': b, 'c': c}) 
df.columns = [list(['WT', 'MUT', 'WTxMUT']), list(['Parent', 'Parent', 'Offspring'])]
df.columns.names = ['Genotype', 'Status']
df_melt = pd.melt(df) 

ax = sb.swarmplot(data = df_melt, x = "Status", y = "value", hue = "Genotype")

def getdatalim(coll):
    x,y = np.array(coll.get_offsets()).T
    try:
        print 'xmin={}, xmax={}, ymin={}, ymax={}'.format(
                x.min(), x.max(), y.min(), y.max())
        rect = Rectangle((x.min(),y.min()),x.ptp(),y.ptp(),edgecolor='k',facecolor='None',lw=3)
        ax.add_patch(rect)
    except ValueError:
        pass

getdatalim(ax.collections[0]) # "Parent"
getdatalim(ax.collections[1]) # "Offspring"

plt.show()

which prints:

xmin=-0.107313729132, xmax=0.10661092707, ymin=-0.598534246847, ymax=0.980441247759
xmin=0.942829146473, xmax=1.06105941656, ymin=0.761277608688, ymax=1.74729717464

And here's the figure: