Minimum length of edges in a graph (that is a tree)

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One way is to set the figsize optional parameter in show to a large value such as 20 or more. Indeed, some lengths (such as distances between nodes) scale with the figure size whereas others (such as font size and line thickness) don't.

One can also play with the graph plotting algorithm; in this case, one could use 'tree' or 'planar' in addition to 'spring', and see if they result in better spread out vertices ('tree' is a good choice here).

Example:

msg_list =  [
    ('F6', 'V3'), ('V3', 'F0'), ('F7', 'V4'), ('V4', 'F1'),
    ('F8', 'V5'), ('V5', 'F2'), ('F9', 'V6'), ('V6', 'F3'),
    ('F10', 'V7'), ('V7', 'F4'), ('F11', 'V8'), ('V8', 'F5'),
    ('V0', 'F0'), ('V1', 'F1'), ('V2', 'F2'), ('S9', 'F0'),
    ('F0', 'S10'), ('S10', 'F1'), ('F1', 'S11'), ('S11', 'F2'),
    ('F2', 'S12'), ('S12', 'F3'), ('F3', 'S13'), ('S13', 'F4'),
    ('F4', 'S14'), ('S14', 'F5'), ('F5', 'S15')
]

c =  {
    '#BBBBBB': ['F6', 'F7', 'F8', 'F9', 'F10', 'F11'],
    '#8888FF': ['V0', 'V1', 'V2', 'V3', 'V4', 'V5', 'V6', 'V7', 'V8'],
    '#FF8888': ['S9', 'S10', 'S11', 'S12', 'S13', 'S14', 'S15'],
    '#88FF88': ['F0', 'F1', 'F2', 'F3', 'F4', 'F5']
}

G = DiGraph([], loops=False, multiedges=False)
msg_nr = 1

for v_from, v_to in msg_list:
    G.add_edge((v_from, v_to), label=f"{msg_nr} ... {msg_nr}")
    msg_nr += 1

opt = {'edge_labels': True, 'vertex_colors': c, 'vertex_size': 500}

p = G.plot(layout='planar', **opt)
p.show(figsize=25)

s = G.plot(layout='spring', **opt)
s.show(figsize=25)

t = G.plot(layout='tree', **opt)
t.show(figsize=25)

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