Saturday, September 27, 2014

A matplotlib example

The following matplotlib script shows some cases where the matplotlib defaults were not sufficient for the job, and how to customize the relevant properties. The main tweaks were:
  • changing the font type from Type 3 to True Type
  • scaling the y values
  • using latex syntax in the labels
  • changing the fontsize for axis labels, axis ticks, and legends
  • two legends
  • using proxy objects (lines or patches) for the legends
  • using axis coordinates for locating the legend
  • semi-transparent legends and grid lines
Here is the python code and the resulting figure:

import matplotlib.pyplot as plt
import matplotlib as mpl
import matplotlib.patches as mpatches
import numpy as np
import json
import sys
import os

# change font type to True Type to avoid Type 3 fonts
# (which are not allowed by some conferences)

mpl.rcParams['pdf.fonttype'] = 42

# x = 1-d array with x values
# ys = six 1-d arrays with y values (one for each line we want to plot).
#      Of these 3 are of one kind, and the other 3 are of another kind.
# xlabel, ylabel = labels for the x and y axis

x, ys, xlabel, ylabel = get_plot_info(datafile)

# I want to scale the y values so that the y axis is easier to read.

ys = ys / 10   # ys is a numpy array

# The plot will be shrunk when it is included in the paper, so that the 
# default fontsize becomes too small. Select a larger fontsize globally.

fontsize = 30

# Plot the first three lines in blue with different line styles;
# then plot the other three in green with similar line styles.

plt.plot(x, ys[0], 'b-', lw=3)
plt.plot(x, ys[1], 'b--', lw=3)
plt.plot(x, ys[2], 'b:', lw=3)
plt.plot(x, ys[3], 'g-', lw=3)
plt.plot(x, ys[4], 'g--', lw=3)
plt.plot(x, ys[5], 'g:', lw=3)

# Set axis labels with larger fontsize.
# Mention the scaling done to the y values.

plt.xlabel(xlabel, fontsize=fontsize)
plt.ylabel(ylabel + r' $\div 10$', fontsize=fontsize)  # latex syntax works!

# Increase the fontsize of the axis ticks

ax = plt.gca()
for labx in ax.get_xticklabels():
for laby in ax.get_yticklabels():

# If you want the lines to reach the left and right extremities of the graph,
# reset the x-limits.
ax.set_xlim( (min(x), max(x)) )

# Instead of showing 6 entries in the legend (one for each of the 6 lines),
# we show one legend for the color code, and another for the line style.
# ('MCTM-...' are methods, and 'en' etc. are languages for which we ran
# the method.)

# first legend (for color code)

blue_patch = mpatches.Patch(color='blue')
green_patch = mpatches.Patch(color='green')

# loc=(x,y) are the coordinates of the lower left corner of the legend,
# where (0,0) if the lower left corner of the axes, and (1,1) is the
# upper right corner.
# framealpha is the transparency level (0=transparent, 1=opaque).

leg1 = plt.legend((blue_patch, green_patch), ('MCTM-DSGNP','MCTM-D'), 
                  loc=(.3,.6), fontsize=fontsize, framealpha=.5)

# second legend (for line style)

# plot empty arrays in black (to avoid the colors used above).

l_en, = plt.plot([], [], 'k-', lw=3, label='en')
l_hi, = plt.plot([], [], 'k--', lw=3, label='hi')
l_hir, = plt.plot([], [], 'k:', lw=3, label='hir')
plt.legend((l_en, l_hi, l_hir), ('en','hi', 'hir'), 
                  loc='lower right', fontsize=fontsize, framealpha=.5)

# Add grid lines, but make them semi-transparent (otherwise they appear too
# prominent when the figure is shrunk).


# The large fontsize pushes axis labels out of the figure; but matplotlib
# offers a function to auto-correct this.


# The saving format is chosen automatically based on the file extension.

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