Graphing

Most of the necessary information for learning how to make basic plots in Python is in the IPython notebook, but there are a few tips and tricks that are worth mentioning.

Show

import numpy as np
import matplotlib as plt

x = np.array(range(20))
y = 3 + 0.5 * x + np.random.randn(20)
plt.plot(x, y)

In Python, sometimes, new figures won’t display by default. Python waits until you tell it to display those figures by using the .show() method in matplotlib.pyplot. In most environments you should be able to either use plt.show() as many times as you want, or just at the end of the code, but if you’re running into problems where only the first graph displays then this can be fixed by only using a single plt.show() at the end of your code.

New Figures

plt.figure() will start a new figure.

Hold

• plt.hold(True) will set the figure to add new data rather than overwrite
• plt.hold(False) will set the figure to overwrite the existing data
• plt.hold() will switch back and forth, but this requires that you know the current state so I prefer explictly using True and False.

z = 2 + 0.75 * x + np.random.randn(20)

Making Graphing Easier

Making nice looking graphs can sometimes take a lot of work. There are a couple of modules that make this easier.

import pandas as pd
import seaborn as sns

data = pd.DataFrame(x, columns=["x"])
data["y"] = y

sns.lmplot('x', 'y', data)
plt.show()

ggplot for Python