[FIXED] overlapping transparent edges in networkx

Issue

I have a graph in networkx with many edges that will overlap. I'm attempting to make them transparent so that the accumulation of edges will build up a darker colour, but however many edges are overlapping they are still the same shade of pale. For example with one edge:

import networkx as nx
import matplotlib.pyplot as plt
import matplotlib.lines as mlines

G = nx.Graph()

colorlist = []
G.add_edge(1,2)

# explicitly set positions
nodes = {1: (1,0),
2: (0,1)}

nx.draw_networkx(G, nodes, alpha=0.1)
ax = plt.gca()
plt.axis("off")
plt.show()

this gives a faint line

but when six transparent lines are layered over each other

import networkx as nx
import matplotlib.pyplot as plt
import matplotlib.lines as mlines

G = nx.Graph()

colorlist = []
G.add_edge(1,2)
G.add_edge(1,2)
G.add_edge(1,2)
G.add_edge(1,2)
G.add_edge(1,2)
G.add_edge(1,2)

# explicitly set positions
nodes = {1: (1,0),
2: (0,1)}

nx.draw_networkx(G, nodes, alpha=0.1)
ax = plt.gca()
plt.axis("off")
plt.show()

it looks the exact same

why does layering transparent lines over each other not make a darker line, and how can I make this happen?

Solution

You need to use MultiGraph instead of Graph. Otherwise, your graph will still contain only one edge, even if you add it multiple times.

The following will create the desired image with "darker" edges:

G = nx.MultiGraph()

colorlist = []
G.add_edge(1,2)
G.add_edge(1,2)
G.add_edge(1,2)
G.add_edge(1,2)
G.add_edge(1,2)
G.add_edge(1,2)

# explicitly set positions
nodes = {1: (1,0),
2: (0,1)}

nx.draw_networkx(G, nodes, alpha=0.1)
ax = plt.gca()
plt.axis("off")
plt.show()

Answered By - Sparky05