How to make labels appear when hovering over a point in multiple axis?

Question

I'm trying to show hover label on matplotlib with multiple axis.

I'm using python 3.6.8 with matplotlib 3.0.3

My plot have multiple axis, and I looked on the example from this post:

Possible to make labels appear when hovering over a point in matplotlib?

But nothing happen (cant see the labels).

Here is my code:

import matplotlib.pyplot as plt
import numpy as np; np.random.seed(1)

x = np.sort(np.random.rand(15))
y = np.sort(np.random.rand(15))
y2 = np.sort(np.random.rand(15))

fig = plt.figure()
ax1 = plt.subplot(2, 2, 1)
line, = plt.plot(x,y)
ax1.grid(True)

ax2 = ax1.twinx()
ax2.plot(x, y2, color='green')
ax2.tick_params(axis='y', labelcolor='green')

annot = ax1.annotate("", xy=(0,0), xytext=(-20,20),textcoords="offset points",
                    bbox=dict(boxstyle="round", fc="w"),
                    arrowprops=dict(arrowstyle="->"))
annot.set_visible(False)

def update_annot(ind):
    x,y = line.get_data()
    annot.xy = (x[ind["ind"][0]], y[ind["ind"][0]])
    text = "x = {}\ny= {}".format(x[ind["ind"][0]], y[ind["ind"][0]])
    annot.set_text(text)
    annot.get_bbox_patch().set_alpha(0.4)


def hover(event):
    vis = annot.get_visible()
    if event.inaxes == ax1:
        cont, ind = line.contains(event)
        if cont:
            update_annot(ind)
            annot.set_visible(True)
            fig.canvas.draw_idle()
        else:
            if vis:
                annot.set_visible(False)
                fig.canvas.draw_idle()

fig.canvas.mpl_connect("motion_notify_event", hover)

plt.show()

when I disable (comments) the second axis (ax2) I can see the labels.

How can I display the hover labels when using multiple axis ?

Answer 1

The problem is indeed that events are triggered only for one of the twin axes. So if you want to label content of both axes, your need to multiplex the labeling, i.e. create two annotations, one for each axes, and adjust the code such that both annotations are potentially visible.

This could look as follows:

import matplotlib.pyplot as plt
import numpy as np; np.random.seed(1)

x = np.sort(np.random.rand(15))
y = np.sort(np.random.rand(15))
y2 = np.sort(np.random.rand(15))

fig = plt.figure()
ax1 = plt.subplot(2, 2, 1)
line1, = plt.plot(x,y)
ax1.grid(True)

ax2 = ax1.twinx()
line2, = ax2.plot(x, y2, color='green')
ax2.tick_params(axis='y', labelcolor='green')

annots = []
for ax in [ax1, ax2]:
    annot = ax1.annotate("", xy=(0,0), xytext=(-20,20),textcoords="offset points",
                        bbox=dict(boxstyle="round", fc="w", alpha=0.4),
                        arrowprops=dict(arrowstyle="->"))
    annot.set_visible(False)
    annots.append(annot)

annot_dic = dict(zip([ax1, ax2], annots))
line_dic = dict(zip([ax1, ax2], [line1, line2]))

def update_annot(line, annot, ind):
    x,y = line.get_data()
    annot.xy = (x[ind["ind"][0]], y[ind["ind"][0]])
    text = "x = {}\ny= {}".format(x[ind["ind"][0]], y[ind["ind"][0]])
    annot.set_text(text)

def hover(event):

    if event.inaxes in [ax1, ax2]:
        for ax in [ax1, ax2]:
            cont, ind = line_dic[ax].contains(event)
            annot = annot_dic[ax]
            if cont:
                update_annot(line_dic[ax], annot, ind)
                annot.set_visible(True)
                fig.canvas.draw_idle()
            else:
                if annot.get_visible():
                    annot.set_visible(False)
                    fig.canvas.draw_idle()

fig.canvas.mpl_connect("motion_notify_event", hover)

plt.show()