Matplotlib: How To Set Ticks Of Twinned Axis In Log Plot

In my plot, a secondary x axis is used to display the value of another variable for some data. Now, the original axis is log scaled. Unfortunaltely, the twinned axis puts the ticks

Solution 1:

This has been asked a while ago, but I stumbled over it with the same question.

I eventually managed to solve the problem by introducing a logscaled (semilogx) transparent (alpha=0) dummy plot.

Example:

import numpy as np
import matplotlib.pyplot as plt

def conversion_func(x):  # some arbitrary transformation function
    return 2 * x**0.5        # from x to z

x = np.logspace(0, 5, 100)
y = np.sin(np.log(x))

fig = plt.figure()

ax = plt.gca()
ax.semilogx(x, y, 'k')
ax.set_xlim(x[0], x[-1])  # this is important in order that limits of both axes match
ax.set_ylabel("$y$")
ax.set_xlabel("$x$", color='C0')
ax.tick_params(axis='x', which='both', colors='C0')
ax.axvline(100, c='C0', lw=3)

ticks_x = np.logspace(0, 5, 5 + 1)  # must span limits of first axis with clever spacing
ticks_z = conversion_func(ticks_x)
ax2 = ax.twiny()  # get the twin axis
ax2.semilogx(ticks_z, np.ones_like(ticks_z), alpha=0)  # transparent dummy plot
ax2.set_xlim(ticks_z[0], ticks_z[-1])
ax2.set_xlabel("$z \equiv f(x)$", color='C1')
ax2.xaxis.label.set_color('C1')
ax2.tick_params(axis='x', which='both', colors='C1')
ax2.axvline(20, ls='--', c='C1', lw=3)  # z=20 indeed matches x=100 as desired

fig.show()

In the above example the vertical lines demonstrate that first and second axis are indeed shifted to one another as wanted. x = 100 gets shifted to z = 2*x**0.5 = 20. The colours are just to clarify which vertical line goes with which axis.

Solution 2:

Don't need to cover them, just Eliminate the ticks!

d= [7,9,14,17,35,70];
j= [100,80,50,40,20,10];

plt.figure()
plt.xscale('log')
plt.plot(freq, freq*spec)  #plot some spectrum

ax1 = plt.gca()  #define my first axis 
ax1.yaxis.set_ticks_position('both')
ax1.tick_params(axis='y',which='both',direction='in');
ax1.tick_params(axis='x',which='both',direction='in');

ax2 = ax1.twiny()  #generates second axis (top) 
ax2.set_xlim(ax1.get_xlim());  #same limits
plt.xscale('log')  #make it log

ax2.set_xticks(freq[d]); #my own 'major' ticks OVERLAPS!!! 
ax2.set_xticklabels(j);  #change labels

ax2.tick_params(axis='x',which='major',direction='in'); 
ax2.tick_params(axis='x',which='minor',top=False); #REMOVE 'MINOR' TICKS
ax2.grid()