How to detect which item in a Matplotlib PathCollection has been clicked on?

Question

I can draw a directed network graph using Matplotlib. Now I want to be able to respond to mouse events so that a user can interact with the network. For example, a node could change its colour when the user clicks on it. This is just a noddy example but it illustrates the point. I'd also like to know which node (label) has been clicked on; I'm not really interested in its x,y coordinates in space.

Here's my code so far:

import matplotlib.pyplot as plt
from matplotlib.collections import PathCollection
import networkx as nx

def createDiGraph():
    G = nx.DiGraph()

    # Add nodes:
    nodes = ['A', 'B', 'C', 'D', 'E']
    G.add_nodes_from(nodes)

    # Add edges or links between the nodes:
    edges = [('A','B'), ('B','C'), ('B', 'D'), ('D', 'E')]
    G.add_edges_from(edges)
    return G

G = createDiGraph()

# Get a layout for the nodes according to some algorithm.
pos = nx.layout.spring_layout(G, random_state=779)

node_size = 300
nodes = nx.draw_networkx_nodes(G, pos, node_size=node_size, node_color=(0,0,0.9),
                                edgecolors='black')
# nodes is a matplotlib.collections.PathCollection object
nodes.set_picker(5)
#nodes.pick('button_press_event')
#print(nodes.get_offsets())

nx.draw_networkx_edges(G, pos, node_size=node_size, arrowstyle='->',
                                arrowsize=15, edge_color='black', width=1)

nx.draw_networkx_labels(G, pos, font_color='red', font_family='arial',
                                font_size=10)

def onpick(event):
    #print(event.mouseevent)

    if isinstance(event.artist, PathCollection):
        #nodes = event.artist
        print (event.ind)

fig = plt.gcf()

# Bind our onpick() function to pick events:
fig.canvas.mpl_connect('pick_event', onpick)

# Hide the axes:
plt.gca().set_axis_off()
plt.show()

The network looks like this when plotted:

If I click with my mouse on node C for example, the program prints out [1]; or [3] for node E. Notice the index doesn't correspond to 0 for A, 1 for B, 2 for C, and so on, even though the original nodes were added to the networkx digraph in that order.

So how do I get the value 'C', when I click on node C? And how do I get hold of the object representing C in the figure so I can change its colour?

I've tried playing around with PathCollection.pick but I'm not sure what to pass into it and I'm not sure that's the right method to use anyway.

Answer 1

I eventually arrived at this solution after helpful comments from @apogalacticon.

import matplotlib.pyplot as plt
from matplotlib.collections import PathCollection
import networkx as nx

def createDiGraph():
    G = nx.DiGraph()

    # Add nodes:
    nodes = ['A', 'B', 'C', 'D', 'E']
    G.add_nodes_from(nodes)

    # Add edges or links between the nodes:
    edges = [('A','B'), ('B','C'), ('B', 'D'), ('D', 'E')]
    G.add_edges_from(edges)
    return G

G = createDiGraph()

# Get a layout for the nodes according to some algorithm.
pos = nx.layout.spring_layout(G, random_state=779)

node_size = 300
nodes = nx.draw_networkx_nodes(G, pos, node_size=node_size, node_color=(0,0,0.9),
                                edgecolors='black')
# nodes is a matplotlib.collections.PathCollection object
nodes.set_picker(5)

nx.draw_networkx_edges(G, pos, node_size=node_size, arrowstyle='->',
                                arrowsize=15, edge_color='black', width=1)

nx.draw_networkx_labels(G, pos, font_color='red', font_family='arial',
                                font_size=10)

def onpick(event):

    if isinstance(event.artist, PathCollection):
        all_nodes = event.artist
        ind = event.ind[0] # event.ind is a single element array.
        this_node_name = pos.keys()[ind]
        print(this_node_name)

        # Set the colours for all the nodes, highlighting the picked node with
        # a different colour:
        colors = [(0, 0, 0.9)] * len(pos)
        colors[ind]=(0, 0.9, 0)
        all_nodes.set_facecolors(colors)

        # Update the plot to show the change:
        fig.canvas.draw() # plt.draw() also works.
        #fig.canvas.flush_events() # Not required? See https://stackoverflow.com/a/4098938/1843329

fig = plt.gcf()

# Bind our onpick() function to pick events:
fig.canvas.mpl_connect('pick_event', onpick)

# Hide the axes:
plt.gca().set_axis_off()
plt.show()

When you click on a node its face colour changes to green whilst the other nodes are set to blue:

Unfortunately it seems the nodes in the network are represented by a single matplotlib.artist.Artist object, which happens to be a collection of paths without any artist children. This means you can't get hold of a single node as such to alter its properties. Instead you're forced to update all the nodes, just making sure the properties for the picked node - colour in this case - are different to the others.

Answer 2

It looks like G does not retain the order of your nodes when it is created, however, the order of the nodes appears to be stored in pos. I would recommend the following:

Add to your import statement:

from ast import literal_eval

Define label in nx.draw_networkx_nodes and create the plot within a function update_plot which takes G, pos, and color as arguments:

def update_plot(pos, G, colors):
    # the keys of the pos dictionary contains the labels that you are interested in
    # or label = [*pos]
    nodes = nx.draw_networkx_nodes(G, pos, node_size=node_size, node_color=colors,edgecolors='black', label=list(pos.keys()))
    # nodes is a matplotlib.collections.PathCollection object
    nodes.set_picker(5)
    nx.draw_networkx_edges(G, pos, node_size=node_size, arrowstyle='->', arrowsize=15, edge_color='black', width=1)
    nx.draw_networkx_labels(G, pos, font_color='red', font_family='arial', font_size=10)

Your onpick function should then be:

def onpick(event):
    if isinstance(event.artist, PathCollection):
        #index of event
        ind = event.ind[0]
        #convert the label list from a string back to a list
        label_list = literal_eval(event.artist.get_label())
        print(label_list[ind])
        colors = [(0, 0, 0.9)] * len(pos)
        colors[ind]=(0.9,0,0)
        update_plot(pos, G, colors)

The body of your code is then simply:

G = createDiGraph()
# Get a layout for the nodes according to some algorithm.
pos = nx.layout.spring_layout(G, random_state=779)

node_size = 300
colors=[(0,0,0.9)]*len(pos)
update_plot(pos, G, colors)