How to add a second x-axis in matplotlib

Question

I have a very simple question  I need to have a second x-axis on my plot and I want that this axis has a certain number of tics that correspond to certain position of the first axis   Let s try with an example  Here I am plotting the dark matter mass as a function of the expansion factor  defined as 1  1 z   that ranges from 0 to 1   semilogy 1  1 z  mass acc massive  -  label  DM   xlim 0 1  ylim 1e8 5e12    I would like to have another x-axis  on the top of my plot  showing the corresponding z for some values of the expansion factor  Is that possible  If yes  how can I have xtics ax

User · Answer

You can use twiny to create 2 x-axis scales. For Example:

import numpy as np
import matplotlib.pyplot as plt

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

a = np.cos(2*np.pi*np.linspace(0, 1, 60.))

ax1.plot(range(60), a)
ax2.plot(range(100), np.ones(100)) # Create a dummy plot
ax2.cla()
plt.show()

Ref: http://matplotlib.sourceforge.net/faq/howto_faq.html#multiple-y-axis-scales

Output: enter image description here

User · Answer

From matplotlib 3 1 onwards you may use ax secondary xaxis import numpy as np import matplotlib pyplot as plt  x   np linspace 1 13  num 301  y    np sin x  1 01  3000    Define function and its inverse f   lambda x  1  1 x  g   lambda x  1 x-1  fig  ax   plt subplots   ax semilogy x  y  label  DM    ax2   ax secondary xaxis  quot top quot   functions  f g    ax2 set xlabel  quot 1  x 1  quot   ax set xlabel  quot x quot   plt show

User · Answer

I m taking a cue from the comments in  Dhara s answer  it sounds like you want to set a list of new tick locations by a function from the old x-axis to the new x-axis  The tick function below takes in a numpy array of points  maps them to a new value and formats them   import numpy as np import matplotlib pyplot as plt  fig   plt figure   ax1   fig add subplot 111  ax2   ax1 twiny    X   np linspace 0 1 1000  Y   np cos X 20   ax1 plot X Y  ax1 set xlabel r Original x-axis   X     new tick locations   np array   2   5   9    def tick function X       V   1  1 X      return     3f    z for z in V   ax2 set xlim ax1 get xlim    ax2 set xticks new tick locations  ax2 set xticklabels tick function new tick locations   ax2 set xlabel r Modified x-axis   1  1 X     plt show

User · Answer

I m forced to post this as an answer instead of a comment due to low reputation  I had a similar problem to Matteo  The difference being that I had no map from my first x-axis to my second x-axis  only the x-values themselves  So I wanted to set the data on my second x-axis directly  not the ticks  however  there is no axes set xdata  I was able to use Dhara s answer to do this with a modification   ax2 lines        instead of using   ax2 cla     When in use also cleared my plot from ax1

User · Answer

If You want your upper axis to be a function of the lower axis tick-values you can do as below  Please note  sometimes get xticks   will have a ticks outside of the visible range  which you have to allow for when converting  import matplotlib pyplot as plt  fig  ax1   plt subplots    ax1   fig add subplot 111   ax1 plot range 5   range 5    ax1 grid True   ax2   ax1 twiny   ax2 set xticks  ax1 get xticks     ax2 set xbound ax1 get xbound    ax2 set xticklabels  x   2 for x in ax1 get xticks      title   ax1 set title  quot Upper x-axis ticks are lower x-axis ticks doubled  quot   title set y 1 1  fig subplots adjust top 0 85   fig savefig  quot 1 png quot    Gives

User · Answer

Answering your question in Dhara s answer comments   I would like on the second x-axis these tics   7 8 99  corresponding to the x-axis position 10  30  40  Is that possible in some way   Yes  it is   import numpy as np import matplotlib pyplot as plt  fig   plt figure   ax1   fig add subplot 111   a   np cos 2 np pi np linspace 0  1  60    ax1 plot range 60   a   ax1 set xlim 0  60  ax1 set xlabel  x   ax1 set ylabel  y    ax2   ax1 twiny   ax2 set xlabel  x-transformed   ax2 set xlim 0  60  ax2 set xticks  10  30  40   ax2 set xticklabels   7   8   99     plt show     You ll get

[python] How to add a second x-axis in matplotlib

Examples related to python

Examples related to matplotlib