Matplotlib custom SI-prefix unit tick formatter

You can use UliEngineering’s format_value() to easily make a custom matplotlib tick formatter that formats values with SI-prefixes like k, M, G, T, …

Formatting the Y axis ticks

The following example formats the Y axis in the Unit J (Joule). For example, 100000 would be formatted as 100 kJ.

The formatter function we use is

def format_joules(value, pos=None):
    return format_value(value, 'J')

In order to set the formatter, function, use

# Set our formatter as Y axis formatter
plt.gca().yaxis.set_major_formatter(mtick.FuncFormatter(format_joules))

Example:

import matplotlib.ticker as mtick
from UliEngineering.EngineerIO import format_value
from matplotlib import pyplot as plt

def format_joules(value, pos=None):
    return format_value(value, 'J')

# Set our formatter as Y axis formatter
plt.gca().yaxis.set_major_formatter(mtick.FuncFormatter(format_joules))

Formatting the X axis ticks

In order to format the X axis ticks instead, use the same formatter function but activate it using

# Set our formatter as Y axis formatter
plt.gca().xaxis.set_major_formatter(mtick.FuncFormatter(format_joules))

How to set the number of decimal places

UliEngineering’s format_value() allows you set the decimal places using e.g. significant_digits=4

def format_joules(value, pos=None):
    return format_value(value, 'J', significant_digits=4)

Full example

This example generates the Y axis plot shown above

import matplotlib.ticker as mtick
from UliEngineering.EngineerIO import format_value
from matplotlib import pyplot as plt
plt.style.use("ggplot")
import numpy as np

def format_joules(value, pos=None):
    return format_value(value, 'J')

# Set our formatter as Y axis formatter
plt.gca().yaxis.set_major_formatter(mtick.FuncFormatter(format_joules))

# Generate test data
test_data = np.arange(1, 1.2e6)
plt.plot(test_data)
plt.gcf().set_size_inches(10,5)
plt.savefig("/ram/mpl-si-formatter.svg")