Matplotlib pyplot axes formatter

Question

I have an image:

Here in the y-axis I would like to get 5x10^-5 4x10^-5 and so on instead of 0.00005 0.00004.

What I have tried so far is:

fig = plt.figure()
ax = fig.add_subplot(111)
y_formatter = matplotlib.ticker.ScalarFormatter(useOffset=True)
ax.yaxis.set_major_formatter(y_formatter)

ax.plot(m_plot,densities1,'-ro',label='0.0<z<0.5')
ax.plot(m_plot,densities2, '-bo',label='0.5<z<1.0')


ax.legend(loc='best',scatterpoints=1)
plt.legend()
plt.show() 

This does not seem to work. The document page for tickers does not seem to provide a direct answer.

Rosalinarosalind

Answer 1

You can use matplotlib.ticker.FuncFormatter to choose the format of your ticks with a function as shown in the example code below. Effectively all the function is doing is converting the input (a float) into exponential notation and then replacing the 'e' with 'x10^' so you get the format that you want.

import matplotlib.pyplot as plt
import matplotlib.ticker as tick
import numpy as np

x = np.linspace(0, 10, 1000)
y = 0.000001*np.sin(10*x)

fig = plt.figure()
ax = fig.add_subplot(111)

ax.plot(x, y)

def y_fmt(x, y):
    return '{:2.2e}'.format(x).replace('e', 'x10^')

ax.yaxis.set_major_formatter(tick.FuncFormatter(y_fmt))

plt.show()

If you're willing to use exponential notation (i.e. 5.0e-6.0) however then there is a much tidier solution where you use matplotlib.ticker.FormatStrFormatter to choose a format string as shown below. The string format is given by the standard Python string formatting rules.

...

y_fmt = tick.FormatStrFormatter('%2.2e')
ax.yaxis.set_major_formatter(y_fmt)

...

Jammie

what does the %2.2e in your short solution do? – Rosalinarosalind

%2.2e chooses the format that you want your strings to be in. The numbers choose now many decimal places you want and the e chooses scientific notation. You can find more detail here. – Jammie

the function y_fmt(x,y) has two arguments but I don't see y, used in the function. I have seen this behavior before, what is it called? And how should I read it? – Hydrangea

x is the value and y the index of the tick. You do not need "y" in general. Of course "y" can be used to do formatting based on tick position. – Penalty

if I have 1000, 2000, ...10000, do you know how I can show them as 1K, 2K, 10K? – Endurant

@Endurant write an appropriate function and use tick.FuncFormatter – Alhambra

@Hydrangea From the docs: A function can also be used directly as a formatter. The function must take two arguments: x for the tick value and pos for the tick position, and must return a str This creates a FuncFormatter automatically. – Tephra

Answer 2

Just a brief modification to the solution for better string formatting: I would recommend changing the format function to include latex formatting:

def y_fmt(x, y):
    return '${:2.1e}'.format(x).replace('e', '\\cdot 10^{') + '}$'

Oleaginous