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

1. make the y-axis from Y to Y/10

2. 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.

Answer 1

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()