Section 1: matplotlib

Table of Contents > Chapter 7 > Section 1

The matplotlib library provides functions that allow you to design programs that produce publication quality plots. Plots can be displayed in a window, where they can be manipulated interactively, or saved directly to a file. We will not attempt to cover the many different kinds of plots that can be generated but will focus on a function that produces two-dimensional line plots:

matplotlib.pyplot.plot(*args,
          **kwargs)

Plot lines and/or markers to the Axes. args is a variable length argument, allowing for multiple x, y pairs with an optional format string. (http://matplotlib.org/api/pyplot_api.html)

Notice the use of *args and **kwargs to specify the parameters, as described in Chapter 5 Section 2. The (rather lengthy) documentation for this function must be read carefully if we are to learn more about the arguments that must be passed when the function is called. *args is used to specify, possibly multiple, sets of coordinates for plotting while **kwargs are optional keyword arguments used to specify properties of the lines to be drawn between data points.

The matplotlib.pyplot package contains many other functions for manipulating plots, including:

matplotlib.pyplot.show(*

args,
          **kwargs)

This function in fact takes only one experimental keyword argument (see http://matplotlib.org/api/pyplot_api.html for details).

and

matplotlib.pyplot.savefig(*args,
          **kwargs)

The documentation for this function specifies that it does in fact take a single required argument in addition to a number of optional keyword arguments. The required argument is a string that specifies the path to and name of the file to which the plot is to be written. The optional keyword arguments allow you to specify properties such as the resolution, background colour and transparency of the figure (see http://matplotlib.org/api/pyplot_api.html for details).

The following function, for example, consumes a list of times, a list of positions and the name of a file. It generates a plot of position against time using a solid green line to connect data points and labels the axes appropriately. The function then saves the plot to the specified file.

import matplotlib.pyplot as pyplt

def plot_posn(lot, lop, fname):
    """
    (listof Real), (listof Real), str -> NoneType

    Effect: generates a plot of positions lop against
    times lot and saves plot to file with name fname.
    Data points are connected using a solid green line.
    """
    pyplt.plot(lot, lop, 'g-')
    pyplt.xlabel('Time (s)')
    pyplt.ylabel('Position (m)')
    pyplt.savefig(fname)
It is recommend that you quickly skim the documentation for the matplotlib.pyplot package to learn about the other options available for controlling the appearance of plots such as drawing tick marks, grid lines and titles.