How to Make a Plot with Two Different Y-axis in Python

Question

I have a 2D array and I use this data to make a plot, like this:

data = [[2.01756016,  2.83041846,  3.81709531,  4.98948707,  6.2480729],
 [2.01756016, 2.59428527, 3.42083514, 4.15525314, 4.87254428],
 [2.01756016, 2.47796649, 2.95297856, 3.37313728, 3.66131579],
 [2.01756016, 2.24718239, 2.56393939, 2.70463483, 2.65250443]]

fig, ax = plt.subplots(figsize=(6, 6))

sidex = np.linspace(0.875, 2.125, 6)
sidey = np.linspace(5, 45, 5)

x, y = np.meshgrid(sidex, sidey)
z = [[data[i][j] for j in range(len(data[0]))] for i in range(len(data))]
plt.pcolormesh(x, y, z)

plt.xticks([1, 1.25, 1.5, 1.75, 2])
plt.yticks([10, 20, 30, 40])

plt.xlabel('X')
plt.ylabel('Y')


plt.show()

Now I am trying to change the y-axis

make the y-axis from Y to Y/10
create second y-axis at the right-hand side of the plot: Y/100

Like the figure below. How can I achieve this?

I try to change plt.yticks([10, 20, 30, 40]) into plt.yticks([1, 2, 3, 4]), but the result is not what I want.

Jan Joswig · Accepted Answer · 2020-06-16 11:06:01Z

2

You can achieve it using twinx(). I recommend to use ax.set_yticks and ax.set_yticklabels for more control, but it would be possible to use plt.yticks(ticks=ticks, labels=labels), too.

EDIT: I added a colorbar to the plot. When creating a colorbar, you can specify a mappable (the object for which the colors should be matched) as the first argument. For this, I assigned the colormesh to a variable for later reuse (image = ax.pcolormesh(x, y, z)).

data = [[2.01756016,  2.83041846,  3.81709531,  4.98948707,  6.2480729],
 [2.01756016, 2.59428527, 3.42083514, 4.15525314, 4.87254428],
 [2.01756016, 2.47796649, 2.95297856, 3.37313728, 3.66131579],
 [2.01756016, 2.24718239, 2.56393939, 2.70463483, 2.65250443]]

fig, ax = plt.subplots(figsize=(6, 6))

ax2 = ax.twinx()

sidex = np.linspace(0.875, 2.125, 6)
sidey = np.linspace(5, 45, 5)

x, y = np.meshgrid(sidex, sidey)
z = [[data[i][j] for j in range(len(data[0]))] for i in range(len(data))]
image = ax.pcolormesh(x, y, z)

ax.set_xticks([1, 1.25, 1.5, 1.75, 2])
ax.set_yticks([10, 20, 30, 40])
ax.set_yticklabels([1, 2, 3, 4])
ax2.set_yticks([10, 20, 30, 40])
ax2.set_yticklabels([10, 20, 30, 40])
ax2.set_ylim(ax.get_ylim())

fig.colorbar(image, pad=0.1)

ax.set_xlabel('X')
ax.set_ylabel('Y/10')
ax2.set_ylabel('Y')
plt.tight_layout()

plt.show()

edited Jun 16, 2020 at 11:06

answered Jun 16, 2020 at 10:13

Jan Joswig

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3 Comments

Joanne Over a year ago

Do you know how to add a color bar in this example? I tried to add "plt.colorbar()" right in front of the "plt.show()", but it doesn't work.

Joanne Over a year ago

Thank you so much for your answer. I have one more question, why "xlabel" doesn't show in the plot?

Jan Joswig Over a year ago

The problem with the plt.something functions is, that they always need to figure out which Axes or Figure to use to draw on. Sometimes the outcome is not what you intended. Maybe this matplotlib tutorial could be interesting to you. I fixed the labels in the plot above now.

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