matplotlib: how to get handles of existing twinx() axes?

Question

I want to create a figure with two y-axes and add multiple curves to each of those axes at different points in my code (from different functions).

In a first function, I create a figure:

   import matplotlib.pyplot as plt
   from numpy import *

   # Opens new figure with two axes
   def function1():
          f = plt.figure(1)
          ax1 = plt.subplot(211)
          ax2 = ax1.twinx()  

          x = linspace(0,2*pi,100)
          ax1.plot(x,sin(x),'b')
          ax2.plot(x,1000*cos(x),'g')

   # other stuff will be shown in subplot 212...

Now, in a second function I want to add a curve to each of the already created axes:

   def function2():
          # Get handles of figure, which is already open
          f = plt.figure(1)
          ax3 = plt.subplot(211).axes  # get handle to 1st axis
          ax4 = ax3.twinx()            # get handle to 2nd axis (wrong?)

          # Add new curves
          x = linspace(0,2*pi,100)
          ax3.plot(x,sin(2*x),'m')
          ax4.plot(x,1000*cos(2*x),'r')

Now my problem is that the green curve added in the first code block is not anymore visible after the second block is executed (all others are).

I think the reason for this is the line

    ax4 = ax3.twinx()

in my second code block. It probably creates a new twinx() instead of returning a handle to the existing one.

How would I correctly get the handles to already existing twinx-axes in a plot?

Answer 1

you can use get_shared_x_axes (get_shared_y_axes) to get a handle to the axes created by twinx (twiny):

# creat some axes
f,a = plt.subplots()

# create axes that share their x-axes
atwin = a.twinx()

# in case you don't have access to atwin anymore you can get a handle to it with
atwin_alt = a.get_shared_x_axes().get_siblings(a)[0]

atwin == atwin_alt # should be True