Network visualization

Version Compatibility

Reference examples tested with: networkx 3.2+, igraph 0.10+ (Python and R), pyvis 0.3+, py4cytoscape 1.9+, matplotlib 3.8+, datashader 0.16+ (for large-graph rasterization).

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show <package> then help(module.function) to check signatures
• R: packageVersion('<pkg>') then ?function_name

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Network Visualization

"Plot a biological network" -> Select a layout algorithm (force-directed for general; hive plot for comparative; ForceAtlas2 for scale-free; circular for small dense), encode node attributes (size by degree/centrality, color by community/module), and choose rendering tier (matplotlib for static publication; PyVis for interactive HTML; Cytoscape for journal-grade compositing). The dominant pitfall is treating layout as biology — node positions in force-directed plots are NOT biologically meaningful; only connectivity is.

• Python: networkx, pyvis.Network, py4cytoscape, datashader (large graphs)
• R: igraph, ggraph (ggplot2-grammar for networks)
• Desktop: Cytoscape (Shannon 2003), Gephi (ForceAtlas2 native)

The Single Most Important Modern Insight -- Layout Is an Artifact, Not Biology

A force-directed layout (Fruchterman-Reingold, ForceAtlas2, spring) is the result of an optimization that minimizes edge crossing and balances repulsion. The visual position of a node has no biological meaning — it is determined by the layout algorithm + random initialization + iteration count + repulsion parameters.

Two consequences:

1. Set random_state / seed for reproducibility. Without it, the same network produces different layouts across runs.
2. Do not read "cluster A is closer to cluster B than C" as biology. Inter-community distances in force-directed layouts are not preserved. Only EDGE existence and node DEGREE are biological signals from the visual.

For biology-faithful layouts, use hive plots (Krzywinski 2012) which anchor nodes to fixed axes by metadata, OR circular layouts which preserve symmetry but don't claim distance meaning.

Decision Tree by Network Type and Question

Network	Recommended layout	Reason
Generic PPI (<500 nodes)	Fruchterman-Reingold OR Kamada-Kawai	General-purpose; clean separation
Scale-free PPI (>500 nodes, hub-spoke)	ForceAtlas2 (Jacomy 2014)	Designed for scale-free networks
Gene regulatory (directed)	Hierarchical OR ForceAtlas2 with edge direction	Direction matters; hierarchical for cascade
Pathway / signaling	Manual or Cytoscape layout	Curated layouts in WikiPathways/Reactome
Co-expression module visualization	Hive plot anchored by module assignment	Comparative; nodes by category
Many-to-many (>10k edges)	Hierarchical edge bundling (Holten 2006)	Reduces visual clutter
Large network (>50k nodes)	Datashader raster + interactive zoom	matplotlib chokes; raster is the only honest display
Connectivity-only (no positions)	Adjacency matrix heatmap	Network as matrix avoids layout artifact
Comparing two networks	Side-by-side same layout (pos reused)	Otherwise layout differences mask biology

Layout Algorithms

import networkx as nx

# Spring / Fruchterman-Reingold (general)
pos = nx.spring_layout(G, k=1/np.sqrt(len(G)), iterations=100, seed=42)

# Kamada-Kawai (better for small dense)
pos = nx.kamada_kawai_layout(G)

# Circular
pos = nx.circular_layout(G)

# Shell (hub at center, periphery outside)
pos = nx.shell_layout(G, nlist=[hub_nodes, periphery_nodes])

# Spectral (reveals clusters)
pos = nx.spectral_layout(G)

# Bipartite (two sets)
pos = nx.bipartite_layout(G, top_nodes)

# Hierarchical (DAG)
pos = nx.nx_pydot.graphviz_layout(G, prog='dot')   # requires graphviz

For ForceAtlas2 in Python: fa2_modified (newer maintained fork) or use Gephi for the canonical implementation. For ggraph in R:

library(ggraph)
ggraph(g, layout = 'fr') +                          # Fruchterman-Reingold
    geom_edge_link(alpha = 0.3) +
    geom_node_point()

ggraph(g, layout = 'kk') +                          # Kamada-Kawai
ggraph(g, layout = 'circle') +
ggraph(g, layout = 'graphopt') +                    # OpenOrd-style for large

Hive Plots (Krzywinski 2012) — Biology-Faithful

A hive plot anchors nodes to 2-3 fixed axes by a categorical attribute (e.g., node type, module, chromosome); edges drawn as arcs between axes. Removes the "hairball" effect by replacing free 2D layout with structured 1D axes.

# HiveNetX or pyveplot for hive layouts
# Or use d3.js HivePlot for interactive
# R: HivePlotData via igraph + custom rendering

Use hive plots when comparing networks across conditions OR when nodes have a categorical structure (e.g., TFs vs targets, chromosomes for 3D-genome interactions).

Hierarchical Edge Bundling (Holten 2006)

For many-to-many networks within a hierarchical structure (gene hierarchies, taxonomies), edge bundling routes edges along the tree backbone, dramatically reducing clutter.

library(ggraph)
ggraph(graph, layout = 'dendrogram', circular = TRUE) +
    geom_conn_bundle(data = get_con(from = from_idx, to = to_idx),
                     alpha = 0.4, tension = 0.8, edge_colour = 'grey60') +
    geom_node_point() +
    theme_void()

NetworkX + matplotlib — Standard Static

Goal: Render a PPI network with node size proportional to degree, color by community, and edge width by interaction confidence.

Approach: Compute layout once with fixed seed; compute attributes (degree, community); render in layers via nx.draw_networkx_* functions for fine control.

import networkx as nx
import matplotlib.pyplot as plt
from networkx.algorithms.community import greedy_modularity_communities
import numpy as np

# Layout with fixed seed for reproducibility
pos = nx.spring_layout(G, k=1.5, seed=42)

# Compute attributes
degrees = dict(G.degree())
communities = list(greedy_modularity_communities(G))
node_to_community = {n: i for i, c in enumerate(communities) for n in c}

# Sizes scaled to degree
sizes = [100 + degrees[n] * 50 for n in G.nodes()]
colors = [node_to_community[n] for n in G.nodes()]

# Render in layers
fig, ax = plt.subplots(figsize=(10, 8))
nx.draw_networkx_edges(G, pos, alpha=0.3, edge_color='grey', width=0.5, ax=ax)
nodes = nx.draw_networkx_nodes(G, pos, node_size=sizes, node_color=colors,
                                cmap='tab20', edgecolors='black', linewidths=0.5, ax=ax)
# Label only high-degree (hub) nodes
hubs = [n for n in G.nodes() if degrees[n] >= 10]
nx.draw_networkx_labels(G, pos, labels={n: n for n in hubs}, font_size=8, ax=ax)
ax.axis('off')
plt.tight_layout()
plt.savefig('network.pdf', bbox_inches='tight', dpi=300)

PyVis — Interactive HTML

from pyvis.network import Network

net = Network(height='700px', width='100%', bgcolor='white', font_color='black')
net.from_nx(G)

# Per-node styling
for node in G.nodes():
    net.get_node(node)['size'] = 10 + degrees[node] * 5
    net.get_node(node)['color'] = palette[node_to_community[node] % len(palette)]
    net.get_node(node)['title'] = f'{node}\nDegree: {degrees[node]}'

net.toggle_physics(True)
net.set_options('{"physics": {"forceAtlas2Based": {"gravitationalConstant": -50}}}')
net.save_graph('network.html')

PyVis wraps vis.js; produces standalone HTML. Suitable for supplementary HTML; not for static journal figure.

Cytoscape Automation (py4cytoscape)

import py4cytoscape as p4c
# Cytoscape desktop must be running

p4c.create_network_from_networkx(G, title='PPI')
p4c.layout_network('force-directed')

# Custom style
style_name = 'DegreeStyle'
p4c.create_visual_style(style_name)
p4c.set_node_size_mapping('degree', [1, 5, 20], [30, 60, 120],
                            mapping_type='c', style_name=style_name)
p4c.set_node_color_mapping('degree', [1, 10, 20], ['#FFFFCC', '#FD8D3C', '#BD0026'],
                             mapping_type='c', style_name=style_name)
p4c.set_visual_style(style_name)

# Export
p4c.export_image('network.pdf', type='PDF')

Cytoscape is the desktop reference for publication-grade biological networks; py4cytoscape exposes script control from Python or R (via cyREST).

Per-Method Failure Modes

Layout positions interpreted as biology

Trigger: "Cluster A is between cluster B and C, so it's transitional."

Mechanism: Force-directed positions are optimization artifacts.

Symptom: Conclusion contradicts orthogonal evidence; not replicable with different seed.

Fix: Frame conclusions in terms of edge existence and node degree only. For trajectory claims, use the relevant time-series tool (RNA velocity, pseudotime), not the network layout.

Layout differs across runs

Trigger: No random seed set.

Mechanism: Spring / FA2 are stochastic.

Symptom: Rerun produces a visibly different figure.

Fix: seed=42 (NetworkX) or set.seed(42) (R igraph) before layout.

Comparing two networks with different layouts

Trigger: spring_layout run separately for two conditions.

Mechanism: Layouts differ; visual change conflated with biological change.

Symptom: Concludes "this protein moved" when only the layout moved.

Fix: Compute layout on the union network OR pass the same pos to both renders.

Hairball — too many edges with poor layout

Trigger: Dense network with default force-directed; >5k edges.

Mechanism: Edge crossings dominate; no structure visible.

Symptom: Visual is a uniform dense blob.

Fix: Hierarchical edge bundling (Holten 2006), filter to top-confidence edges, use a hive plot, OR raster with Datashader.

Hub labels obscure non-hub structure

Trigger: Labeling every node in a network with >100 nodes.

Mechanism: Labels overlap; visual clutter.

Symptom: Cannot read any labels; figure too busy.

Fix: Label only hubs (degree > threshold) OR genes of interest. Use ggrepel-style repulsion in matplotlib via adjustText.

Edge widths uniform when weights are meaningful

Trigger: Default width=1 for all edges.

Mechanism: Edge attribute (correlation, confidence, weight) not encoded.

Symptom: Reader cannot tell strong from weak interactions.

Fix: width = [G[u][v]['weight'] for u, v in G.edges()] with normalization to visible range.

PyVis HTML size explodes for large networks

Trigger: net.from_nx(G) with 10000+ nodes.

Mechanism: Embedded JavaScript file balloons; browser hangs.

Symptom: HTML file 100+ MB; doesn't render.

Fix: For large networks switch to Datashader or Cytoscape with Cytoscape.js for web; PyVis is for <2000 nodes.

Reconciliation: When Layouts Disagree

Pattern	Cause	Action
Two layouts of same network look different	Different algorithm or seed	Standardize; report algorithm + seed
Cytoscape and NetworkX disagree	Cytoscape default = grid; NetworkX = spring	Pick one; document
Communities don't separate visually	Layout doesn't preserve community structure	Use spectral layout OR color-code communities; do not rely on positional separation
Same nodes "move" between conditions	Layout re-computed	Reuse layout from union network

Quantitative Thresholds

Threshold	Value	Source
Max edges for spring layout legibility	~2000	Practical
Max nodes for PyVis HTML	~2000	Browser memory
When to bundle edges	>5000 edges or many-to-many	Holten 2006
When to use Datashader	>50000 nodes or edges	Standard
Min degree for labeling	depends; 5-10 typical	Practical
Random seed	always set (42 is convention)	Reproducibility

Common Errors

Error / symptom	Cause	Solution
Layout differs across runs	No seed	Always seed=42
"Distance between clusters" interpreted	Layout artifact	Frame conclusions on edges/degree only
Hairball	Dense + force-directed	Bundle / hive / filter / Datashader
Two networks' layouts not comparable	Computed separately	Use union network layout
Edge widths uniform	Default	Encode weight
Label clutter	All nodes labeled	Hubs only
PyVis 100MB HTML	Too large for PyVis	Switch to Cytoscape.js / Datashader

References

• Csardi G, Nepusz T. 2006. The igraph software package for complex network research. InterJournal Complex Systems 1695.
• Fruchterman TMJ, Reingold EM. 1991. Graph drawing by force-directed placement. Softw Pract Exp 21(11):1129-1164.
• Hagberg A, Schult D, Swart P. 2008. Exploring network structure, dynamics, and function using NetworkX. Proc 7th Python in Science Conference (SciPy 2008).
• Holten D. 2006. Hierarchical edge bundles: visualization of adjacency relations in hierarchical data. IEEE TVCG 12(5):741-748.
• Jacomy M, Venturini T, Heymann S, Bastian M. 2014. ForceAtlas2, a continuous graph layout algorithm for handy network visualization designed for the Gephi software. PLoS ONE 9(6):e98679.
• Krzywinski M, Birol I, Jones SJM, Marra MA. 2012. Hive plots—rational approach to visualizing networks. Brief Bioinform 13(5):627-644.
• Pedersen T. 2024. ggraph (CRAN). https://ggraph.data-imaginist.com
• Shannon P, et al. 2003. Cytoscape: a software environment for integrated models of biomolecular interaction networks. Genome Res 13(11):2498-2504.

Related Skills

• gene-regulatory-networks/coexpression-networks - Build the network to visualize
• database-access/interaction-databases - Fetch PPI data
• data-visualization/multipanel-figures - Combine network with other plots
• data-visualization/color-palettes - Community / module color schemes
• single-cell/cell-communication - Cell-cell interaction networks