Dealing with plotting negative, zero and positive values in log scale

Posted on Wed 30 March 2016 in Misc

Sometimes you have to show positive, zero and negative number in log scale. However you cannot take log of negative numbers and zero. But one could approximate it with a log transform modulus as stated here.

In Python with numpy:

from numpy import sign, abs, log10  
import matplotlib.pyplot as plt

# Data varies in several magnitudes and has both positive, zero and negative numbers  
x = [-10000,-1000,-100,-10,0,10,100,1000,10000]  
# log modulus transform  
x_log_modulus_transform= sign(x)*(log10(abs(x)+1))

f, ax = plt.subplots(2, sharex=True)  
ax[0].plot(x,'o')  
ax[0].margins(x=0.12, y=0.2) # for better visualization of datapoints at the end of axis

ax[1].plot(x_log_modulus_transform,'o')  
ax[1].margins(x=0.12, y=0.2) # for better visualization of datapoints at the end of axis  
ax[0].set_ylabel('x')  
ax[1].set_ylabel('sign(x)*(log(|x|+1))')  
plt.show()

Python