labs/Module-2/7.Matplot/2-Simple-Line-Plots.ipynb

Simple Line Plots

Perhaps the simplest of all plots is the visualization of a single function y = f(x). Here we will take a first look at creating a simple plot of this type. As in all the following chapters, we'll start by setting up the notebook for plotting and importing the packages we will use:

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%matplotlib inline
import matplotlib.pyplot as plt
plt.style.use('seaborn-whitegrid')
import numpy as np

For all Matplotlib plots, we start by creating a figure and axes. In their simplest form, this can be done as follows (see the following figure):

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fig = plt.figure()
ax = plt.axes()

In Matplotlib, the figure (an instance of the class plt.Figure) can be thought of as a single container that contains all the objects representing axes, graphics, text, and labels. The axes (an instance of the class plt.Axes) is what we see above: a bounding box with ticks, grids, and labels, which will eventually contain the plot elements that make up our visualization. Throughout this part of the book, I'll commonly use the variable name fig to refer to a figure instance and ax to refer to an axes instance or group of axes instances.

Once we have created an axes, we can use the ax.plot method to plot some data. Let's start with a simple sinusoid, as shown in the following figure:

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fig = plt.figure()
ax = plt.axes()

x = np.linspace(0, 10, 1000)
ax.plot(x, np.sin(x));

Note that the semicolon at the end of the last line is intentional: it suppresses the textual representation of the plot from the output.

Alternatively, we can use the PyLab interface and let the figure and axes be created for us in the background (see Two Interfaces for the Price of One for a discussion of these two interfaces); as the following figure shows, the result is the same:

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plt.plot(x, np.sin(x));

If we want to create a single figure with multiple lines (see the following figure), we can simply call the plot function multiple times:

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plt.plot(x, np.sin(x))
plt.plot(x, np.cos(x));

That's all there is to plotting simple functions in Matplotlib! We'll now dive into some more details about how to control the appearance of the axes and lines.

Adjusting the Plot: Line Colors and Styles

The first adjustment you might wish to make to a plot is to control the line colors and styles. The plt.plot function takes additional arguments that can be used to specify these. To adjust the color, you can use the color keyword, which accepts a string argument representing virtually any imaginable color. The color can be specified in a variety of ways; see the following figure for the output of the following examples:

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plt.plot(x, np.sin(x - 0), color='blue')        # specify color by name
plt.plot(x, np.sin(x - 1), color='g')           # short color code (rgbcmyk)
plt.plot(x, np.sin(x - 2), color='0.75')        # grayscale between 0 and 1
plt.plot(x, np.sin(x - 3), color='#FFDD44')     # hex code (RRGGBB, 00 to FF)
plt.plot(x, np.sin(x - 4), color=(1.0,0.2,0.3)) # RGB tuple, values 0 to 1
plt.plot(x, np.sin(x - 5), color='chartreuse'); # HTML color names supported

If no color is specified, Matplotlib will automatically cycle through a set of default colors for multiple lines.

Similarly, the line style can be adjusted using the linestyle keyword (see the following figure):

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plt.plot(x, x + 0, linestyle='solid')
plt.plot(x, x + 1, linestyle='dashed')
plt.plot(x, x + 2, linestyle='dashdot')
plt.plot(x, x + 3, linestyle='dotted');

# For short, you can use the following codes:
plt.plot(x, x + 4, linestyle='-')  # solid
plt.plot(x, x + 5, linestyle='--') # dashed
plt.plot(x, x + 6, linestyle='-.') # dashdot
plt.plot(x, x + 7, linestyle=':');  # dotted

Though it may be less clear to someone reading your code, you can save some keystrokes by combining these linestyle and color codes into a single non-keyword argument to the plt.plot function; the following figure shows the result:

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plt.plot(x, x + 0, '-g')   # solid green
plt.plot(x, x + 1, '--c')  # dashed cyan
plt.plot(x, x + 2, '-.k')  # dashdot black
plt.plot(x, x + 3, ':r');  # dotted red