Few days back I had an assignment to visualize a network of people who talk to each other. I needed to find the people who are contacted most. Drawing a network graph seemed like the best way to find it out visually.

So I looked around for tools that could help with it and came across Networkx. They have a nice documentation and a Lot of examples on drawing different kinds of graphs. I decided to try it out. Here’s how to draw a simple undirected graph with it -

import networkx as nx
import matplotlib.pyplot as plt

def draw_graph(graph):
    # create networkx graph
    G=nx.Graph()

    # add edges
    for edge in graph:
        G.add_edge(edge[0], edge[1])

    # There are graph layouts like shell, spring, spectral and random.
    # Shell layout usually looks better, so we're choosing it.
    # I will show some examples later of other layouts
    graph_pos = nx.shell_layout(G)

    # draw nodes, edges and labels
    nx.draw_networkx_nodes(G, graph_pos, node_size=1000, node_color='blue', alpha=0.3)
    nx.draw_networkx_edges(G, graph_pos)
    nx.draw_networkx_labels(G, graph_pos, font_size=12, font_family='sans-serif')

    # show graph
    plt.show()

# draw example
# graph is a list of tuples of nodes. Each tuple defining the
# connection between 2 nodes
graph = [(20, 21), (21, 22), (22, 23), (23, 24), (24, 25), (25, 20)]

draw_graph(graph)

And this is the image that’s produced -

We can try adding some more edges and see how it looks. E.g. -

graph = [
    (20, 21), (21, 22), (22, 23), (23, 24), (24, 25), (25, 20),
    (25, 21), (23, 20), (24, 22)
]

This generates -

Neat!

Directed Graph

But the above graph is undirected. I needed directed to know which node is receiving more connections from others. Then I found out that Networkx has a Graph class called DiGraph, which can be used to draw directed graphs. Let’s try that -

def draw_graph(graph):
    # create directed networkx graph
    G=nx.DiGraph()

    # add edges
    G.add_edges_from(graph)

    graph_pos = nx.shell_layout(G)

    # draw nodes, edges and labels
    nx.draw_networkx_nodes(G, graph_pos, node_size=1000, node_color='blue', alpha=0.3)
    # we can now added edge thickness and edge color
    nx.draw_networkx_edges(G, graph_pos, width=2, alpha=0.3, edge_color='green')
    nx.draw_networkx_labels(G, graph_pos, font_size=12, font_family='sans-serif')

    # show graph
    plt.show()

# we can add more edges here as the direction is a factor now,
# edges are added as (from_node, to_node) tuples
# hence (22, 25) and (25, 22) are different. In undirected graph,
# we couldn't have told the difference.
graph = graph = [
        (20, 21), (21, 22), (22, 23), (23, 24), (24, 25), (25, 20),
        (25, 21), (23, 20), (24, 22), (21, 24), (20, 21)
    ]
draw_graph(graph)

This generates -

That works great! But we’re still missing one thing, a label for the edge.

labels = range(len(graph))
# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

edge_labels = dict(zip(graph, labels))
# {(23, 20): 7, (22, 23): 2, (20, 21): 10, (24, 25): 4, (21, 22): 1, (25, 20): 5, (24, 22): 8, (25, 21): 6, (23, 24): 3, (21, 24): 9}

nx.draw_networkx_edge_labels(G, graph_pos, edge_labels=edge_labels)

# show graph
plt.show()

Which generates -

You can follow the same data structure and add any label you want to the edges.

Layouts

Now let’s try out the other layouts we mentioned -

Spring layout

graph_pos = nx.spring_layout(G)

Spectral layout

graph_pos = nx.spectral_layout(G)

Random layout

graph_pos = nx.random_layout(G)

Overall, I am liking Networkx. There are many more types of graphs that can be drawn. Can’t wait to try them.

Bibhas Debnath

iambibhas bibhasdn


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