CODEWITHSUNDEEP
pythonmatplotlib
Luk
Luk

Reputation: 1149

Python: Draw a second y-axis on pyplot figure

I am trying to create a subplot that consists of two figures. Each of the figures shows some data plotted vs a time axis. And for each figure I want to have two y axes corresponding to two different graphs shown within the same figure.

Let's start with the data corresponding to one of the y-axes. This data is the same for each of the two figures and is generated as follows (it is fairly ugly code and if you have any suggestions as on how to improve it, please let me know!):

pwm_len = len(Time)/6
pwm_max = 255
pwm_min = 150
pwm_mid = 200
pwm_zero = 0
pwm1 = np.repeat(pwm_max, pwm_len) 
pwm2 = np.repeat(pwm_min, pwm_len)
pwm3 = np.repeat(pwm_max, pwm_len)
pwm4 = np.repeat(pwm_mid, pwm_len)
pwm5 = np.repeat(pwm_max, pwm_len)
pwm6 = np.repeat(pwm_zero, pwm_len)
pwm = pwm1 + pwm2 + pwm3 + pwm4 + pwm5 + pwm6

To create the figure, I am using the following code (please note that it is not working right now, due to some wrong usage of twinx() ):

fig, axs = plt.subplots(2, sharex=True, sharey=True)
plt.subplots_adjust(hspace=0.5)
axs_pwm = axs.twinx()
axs[0].plot(Time, velocity, 'b-')
axs_pwm[0].plot(Time, pwm, 'r-')
axs[0].set_ylabel('[mm/s]')
axs_pwm[0].set_ylabel('PWM')
axs[0].grid(True)
axs[1].plot(Time, velocity_filtered, 'b-')
axs_pwm[1].plot(Time, pwm, 'r-')
axs[1].set_ylabel('[mm/s]')
axs_pwm[1]-set_ylabel('PWM')
axs[1].grid(True)

plt.show()

apparently I am using the twinx() function in a wrong way. But what is a different way to draw the second y axis?

Upvotes: 0

Views: 381

Answers (1)

Sheldore
Sheldore

Reputation: 39052

Extending upon ImportanceOfBeingErnest's's suggestion, you need the following:

  • Create the twin axis for each subplot using the index 0 and 1 while using twinx()
  • Use the respective twin axis' object to plot data and set y-axis labels
fig, axs = plt.subplots(2, sharex=True, sharey=True)
plt.subplots_adjust(hspace=0.5)

axs_pwm1 = axs[0].twinx() # Create twin axis for the first subplot
axs[0].plot(Time, velocity, 'b-')
axs_pwm1.plot(Time, pwm, 'r-')
axs[0].set_ylabel('[mm/s]')
axs_pwm1.set_ylabel('PWM')
axs[0].grid(True)

axs_pwm2 = axs[1].twinx() # Create twin axis for the second subplot
axs[1].plot(Time, velocity_filtered, 'b-')
axs_pwm2.plot(Time, pwm, 'r-')
axs[1].set_ylabel('[mm/s]')
axs_pwm2.set_ylabel('PWM')
axs[1].grid(True)

plt.show()

Or as suggested by @SpghttCd in the comments, you can predefine all the twin axis and then use index as 

ax2 = [ax.twinx() for ax in axs]

ax2[0].plot(...)
ax2[1].plot(...)

Upvotes: 1

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