Matplotlib Second x-axis with transformed values

Question

I have been using a piece of code (based on the solution to another's problem given here) to create plots of spectroscopic data with two x-axis. The first (bottom) is in frequency units, the second (top) is just transformed to wavelength units (wavelength = 3E8/frequency). This was working well until I upgraded MPL to 1.4.2 after which the values on the upper axis are just the same as those on the lower axis (see example).

A MWE (an exact copy from the MPL mailing list) is:

from matplotlib.transforms import Transform, BlendedGenericTransform, IdentityTransform 
import matplotlib.pyplot as plt 
from mpl_toolkits.axes_grid.parasite_axes import SubplotHost 
import numpy as np 

c = 3.e2 

class Freq2WavelengthTransform(Transform): 
    input_dims = 1 
    output_dims = 1 
    is_separable = False 
    has_inverse = True 

    def transform(self, tr): 
        return c/tr 

    def inverted(self): 
        return Wavelength2FreqTransform() 


class Wavelength2FreqTransform(Freq2WavelengthTransform): 
    def inverted(self): 
        return Freq2WavelengthTransform() 

aux_trans = BlendedGenericTransform(Freq2WavelengthTransform(), 
IdentityTransform()) 

fig = plt.figure(2) 

ax_GHz = SubplotHost(fig, 1,1,1) 
fig.add_subplot(ax_GHz) 
ax_GHz.set_xlabel("Frequency (GHz)") 


xvals = np.arange(199.9, 999.9, 0.1) 
#make some test data 
data = np.sin(0.03*xvals) 

ax_mm = ax_GHz.twin(aux_trans) 
ax_mm.set_xlabel('Wavelength (mm)') 
ax_mm.set_viewlim_mode("transform") 
ax_mm.axis["right"].toggle(ticklabels=False) 

ax_GHz.plot(xvals, data) 
ax_GHz.set_xlim(200, 1000) 

plt.draw() 
plt.show()

This produces MWE output

Can any one advise me how to address this in MPL 1.4.2?

Possible duplicate of [How to add a second x-axis in matplotlib](http://stackoverflow.com/q/10514315/2823755). Maybe [the accepted answer](http://stackoverflow.com/a/10517481/2823755) will help. [```pyplot.twiny()```](http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.twiny) — wwii, Jan 23 '15 at 14:52

score 12 · Answer 1 · edited May 23 '17 at 12:02

Using a combination of Adobe's answer from the thread linked to in wwii's comment, and your own code.

import numpy as np
import matplotlib.pyplot as plt

c=3.e2
fig = plt.figure()    
ax1 = fig.add_subplot(111)    
ax2 = ax1.twiny()

xvals = np.arange(199.9, 999.9, 0.1)     
data = np.sin(0.03*xvals)     
ax1.plot(xvals, data)

ax1Ticks = ax1.get_xticks()   
ax2Ticks = ax1Ticks

def tick_function(X):
    V = c/X
    return ["%.3f" % z for z in V]

ax2.set_xticks(ax2Ticks)
ax2.set_xbound(ax1.get_xbound())
ax2.set_xticklabels(tick_function(ax2Ticks))

ax1.set_xlabel("Frequency (GHz)") 
ax2.set_xlabel('Wavelength (mm)')
ax1.grid(True)
plt.ylim(ymin=-1.1,ymax=1.1)
plt.show()

This produces; Output

I hope this helps!

Matplotlib Second x-axis with transformed values

1 Answers1