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04.14 Visualization With Seaborn

IPythonPandasData scienceO'ReillyNumpyscikit-learnpythonMatplotlib
https://picsum.photos/seed/9a275fea5e864b22830903969c7aa284/600/800
Matplotlib has proven to be an incredibly useful and popular visualization tool, but even avid users will admit it often leaves much to be desired. There are several valid complaints about Matplotlib that often come up:
  • Prior to version 2.0, Matplotlib's defaults are not exactly the best choices. It was based off of MATLAB circa 1999, and this often shows.
  • Matplotlib's API is relatively low level. Doing sophisticated statistical visualization is possible, but often requires a lot of boilerplate code.
  • Matplotlib predated Pandas by more than a decade, and thus is not designed for use with Pandas DataFrames. In order to visualize data from a Pandas DataFrame, you must extract each Series and often concatenate them together into the right format. It would be nicer to have a plotting library that can intelligently use the DataFrame labels in a plot.
An answer to these problems is Seaborn. Seaborn provides an API on top of Matplotlib that offers sane choices for plot style and color defaults, defines simple high-level functions for common statistical plot types, and integrates with the functionality provided by Pandas DataFrames.
To be fair, the Matplotlib team is addressing this: it has recently added the plt.style tools discussed in Customizing Matplotlib: Configurations and Style Sheets, and is starting to handle Pandas data more seamlessly. The 2.0 release of the library will include a new default stylesheet that will improve on the current status quo. But for all the reasons just discussed, Seaborn remains an extremely useful addon.

Seaborn Versus Matplotlib

Here is an example of a simple random-walk plot in Matplotlib, using its classic plot formatting and colors. We start with the typical imports:
python
import matplotlib.pyplot as plt
plt.style.use('classic')
%matplotlib inline
import numpy as np
import pandas as pd
Now we create some random walk data:
python
# Create some data
rng = np.random.RandomState(0)
x = np.linspace(0, 10, 500)
y = np.cumsum(rng.randn(500, 6), 0)
And do a simple plot:
python
# Plot the data with Matplotlib defaults
plt.plot(x, y)
plt.legend('ABCDEF', ncol=2, loc='upper left');
png
Although the result contains all the information we'd like it to convey, it does so in a way that is not all that aesthetically pleasing, and even looks a bit old-fashioned in the context of 21st-century data visualization.
Now let's take a look at how it works with Seaborn. As we will see, Seaborn has many of its own high-level plotting routines, but it can also overwrite Matplotlib's default parameters and in turn get even simple Matplotlib scripts to produce vastly superior output. We can set the style by calling Seaborn's set() method. By convention, Seaborn is imported as sns:
python
import seaborn as sns
sns.set()
Now let's rerun the same two lines as before:
python
# same plotting code as above!
plt.plot(x, y)
plt.legend('ABCDEF', ncol=2, loc='upper left');
png
Ah, much better!

Exploring Seaborn Plots

The main idea of Seaborn is that it provides high-level commands to create a variety of plot types useful for statistical data exploration, and even some statistical model fitting.
Let's take a look at a few of the datasets and plot types available in Seaborn. Note that all of the following could be done using raw Matplotlib commands (this is, in fact, what Seaborn does under the hood) but the Seaborn API is much more convenient.

Histograms, KDE, and densities

Often in statistical data visualization, all you want is to plot histograms and joint distributions of variables. We have seen that this is relatively straightforward in Matplotlib:
python
data = np.random.multivariate_normal([0, 0], [[5, 2], [2, 2]], size=2000)
data = pd.DataFrame(data, columns=['x', 'y'])

for col in 'xy':
    plt.hist(data[col], normed=True, alpha=0.5)
png
Rather than a histogram, we can get a smooth estimate of the distribution using a kernel density estimation, which Seaborn does with sns.kdeplot:
python
for col in 'xy':
    sns.kdeplot(data[col], shade=True)
png
Histograms and KDE can be combined using distplot:
python
sns.distplot(data['x'])
sns.distplot(data['y']);
png
If we pass the full two-dimensional dataset to kdeplot, we will get a two-dimensional visualization of the data:
python
sns.kdeplot(data);
png
We can see the joint distribution and the marginal distributions together using sns.jointplot. For this plot, we'll set the style to a white background:
python
with sns.axes_style('white'):
    sns.jointplot("x", "y", data, kind='kde');
png
There are other parameters that can be passed to jointplot—for example, we can use a hexagonally based histogram instead:
python
with sns.axes_style('white'):
    sns.jointplot("x", "y", data, kind='hex')
png

Pair plots

When you generalize joint plots to datasets of larger dimensions, you end up with pair plots. This is very useful for exploring correlations between multidimensional data, when you'd like to plot all pairs of values against each other.
We'll demo this with the well-known Iris dataset, which lists measurements of petals and sepals of three iris species:
python
iris = sns.load_dataset("iris")
iris.head()
Visualizing the multidimensional relationships among the samples is as easy as calling sns.pairplot:
python
sns.pairplot(iris, hue='species', size=2.5);
png

Faceted histograms

Sometimes the best way to view data is via histograms of subsets. Seaborn's FacetGrid makes this extremely simple. We'll take a look at some data that shows the amount that restaurant staff receive in tips based on various indicator data:
python
tips = sns.load_dataset('tips')
tips.head()
python
tips['tip_pct'] = 100 * tips['tip'] / tips['total_bill']

grid = sns.FacetGrid(tips, row="sex", col="time", margin_titles=True)
grid.map(plt.hist, "tip_pct", bins=np.linspace(0, 40, 15));
png

Factor plots

Factor plots can be useful for this kind of visualization as well. This allows you to view the distribution of a parameter within bins defined by any other parameter:
python
with sns.axes_style(style='ticks'):
    g = sns.factorplot("day", "total_bill", "sex", data=tips, kind="box")
    g.set_axis_labels("Day", "Total Bill");
png

Joint distributions

Similar to the pairplot we saw earlier, we can use sns.jointplot to show the joint distribution between different datasets, along with the associated marginal distributions:
python
with sns.axes_style('white'):
    sns.jointplot("total_bill", "tip", data=tips, kind='hex')
png
The joint plot can even do some automatic kernel density estimation and regression:
python
sns.jointplot("total_bill", "tip", data=tips, kind='reg');
png

Bar plots

Time series can be plotted using sns.factorplot. In the following example, we'll use the Planets data that we first saw in Aggregation and Grouping:
python
planets = sns.load_dataset('planets')
planets.head()
python
with sns.axes_style('white'):
    g = sns.factorplot("year", data=planets, aspect=2,
                       kind="count", color='steelblue')
    g.set_xticklabels(step=5)
png
We can learn more by looking at the method of discovery of each of these planets:
python
with sns.axes_style('white'):
    g = sns.factorplot("year", data=planets, aspect=4.0, kind='count',
                       hue='method', order=range(2001, 2015))
    g.set_ylabels('Number of Planets Discovered')
png
For more information on plotting with Seaborn, see the Seaborn documentation, a [tutorial](http://seaborn.pydata.org/ tutorial.htm), and the Seaborn gallery.

Example: Exploring Marathon Finishing Times

Here we'll look at using Seaborn to help visualize and understand finishing results from a marathon. I've scraped the data from sources on the Web, aggregated it and removed any identifying information, and put it on GitHub where it can be downloaded (if you are interested in using Python for web scraping, I would recommend Web Scraping with Python by Ryan Mitchell). We will start by downloading the data from the Web, and loading it into Pandas:
python
# !curl -O https://raw.githubusercontent.com/jakevdp/marathon-data/master/marathon-data.csv
python
data = pd.read_csv('marathon-data.csv')
data.head()
By default, Pandas loaded the time columns as Python strings (type object); we can see this by looking at the dtypes attribute of the DataFrame:
python
data.dtypes
age int64 gender object split object final object dtype: object
Let's fix this by providing a converter for the times:
python
import datetime

def convert_time(s):
    h, m, s = map(int, s.split(':'))
    return datetime.timedelta(hours=h, minutes=m, seconds=s)

data = pd.read_csv('marathon-data.csv',
                   converters={'split':convert_time, 'final':convert_time})
data.head()
python
data.dtypes

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