To plot data we use another module:
matplotlib This is a very powerful (and complicated) plotting library, that be used for quick analysis of experimental data, or to generate publication quality figures. It supports an enormous number of plot types. We are going to start with simple 2D $x,y$ plots.
import matplotlib.pyplot as plt %matplotlib inline import numpy as np
import statement loads up the part of the
matplotlib library we will use for plotting, and lets us refer to this as
plt for convenience later.
%matplotlib inline command tells the Jupyter notebook that we want all out “plots” to appear “inline”, i.e. inside the notebook (alternatives include opening the plots in other windows, or saving them as graphics files). The
% symbol at the start means this is a “magic” command for controlling the behaviour of this Jupyter notebook, and is not standard Python.
If you are using a high resolution screen, you will also want to switch on high resolution figures.
%config InlineBackend.figure_format = 'retina'
We also import
np so that we can store our data as arrays.
Creating a plot uses
plt.plot(). Remember, we have assigned
plt as shorthand for
# plot the numpy arrays a and b against each other import numpy as np a = np.array( [ 1, 2, 3, 4 ] ) b = np.array( [ 5, 6, 7, 8 ] ) print( "a:", a ) print( "b:", b ) plt.plot( a, b ) plt.show()
This can be used for plotting $y$ as a function of $x$, e.g. $y=x^2$.
x = np.array( [0, 1, 2, 3, 4, 5] ) y = x**2 plt.plot( x, y ) plt.show()
The default plot shows a connected line. To plot individual points, we can add a third argument to
plt.plot() that specifies the appearance for that data set:
plt.plot( x, y, "o" ) plt.show()
We can also control the line style, and combine code controlling marker and line appearance.
plt.plot( x, y, ":" ) # dotted line plt.show()
plt.plot( x, y, "s:" ) # dotted line with squares plt.show()
Adding axes labels and a title uses the
Plotting multiple data sets on the same graph uses multiple
plot() commands. For an example, let us create three
# create three numpy arrays, u, v, and w u = x + 1 v = x ** 2 w = np.sqrt( (x*2)+1 ) print('u = ',u) print('v = ',v) print('w = ',w)
Now we can plot $u$, $v$, and $w$ versus $x$ on the same figure.
plt.plot( x, u, 'o-', label='x+1' ) plt.plot( x, v, 'x--', label='x**2' ) plt.plot( x, w, '*:', label='sqrt((x*2)+1)' ) plt.xlabel( 'x' ) plt.ylabel( 'y' ) plt.title( 'y=f(x)') plt.legend() plt.show()
We have assigned text labels for each data set by setting
label=string in each
plt.plot() command. These labels are then shown in the legend produced by the
Nearly every part of the plot appearance can be controlled. Two further examples are line colours and thickness. A number of line colours are predefined and can be referred to with a corresponding string.
# run this cell plt.plot( x, u, 'o-', label='x+1', color='salmon', linewidth=3 ) plt.plot( x, v, 'x--', label='x**2', color='darkolivegreen', linewidth=2 ) plt.plot( x, w, '*:', label='sqrt((x*2)+1)', color='slategrey', linewidth=4 ) plt.xlabel( 'x' ) plt.ylabel( 'y' ) plt.title( 'Too many options can lead to ugly graphs' ) plt.legend() plt.show()
You can save a figure to an external file using
plt.savefig('filename') instead of