Bio Copy Number Cnv Visualization

Version Compatibility

Reference examples tested with: matplotlib 3.8+, pandas 2.2+, numpy 1.26+, seaborn 0.13+, CNVkit 0.9.10+, GATK 4.5+; R 4.3+ with ggplot2 3.5+.

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

• Python: pip show matplotlib pandas then help(function) for signatures
• R: packageVersion('ggplot2') then ?function_name
• CLI: cnvkit.py version, gatk --version

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

CNV Visualization

"Plot my copy number profile" -> A CNV figure is an argument, not a picture. The plot type, the y-axis quantity, and where the diploid baseline sits all determine what the reader can conclude. The single most important rule: a depth-only log2 plot cannot show loss of heterozygosity, cannot show tumor purity, and silently misleads if the diploid baseline is centered on a non-diploid mode.

• CLI: cnvkit.py scatter / diagram / heatmap; gatk PlotModeledSegments
• Python: matplotlib for custom genome-wide and allele-specific tracks
• R: ggplot2, karyoploteR for publication ideograms

Plot Selection — What Each View Reveals and Hides

The decision rule: if the biological question involves LOH, allele-specific gain, or whole-genome doubling, a log2-only plot is insufficient — pair it with a BAF track.

CNVkit Built-in Plots

cnvkit.py scatter sample.cnr -s sample.cns -o scatter.png            # genome-wide
cnvkit.py scatter sample.cnr -s sample.cns -c chr17 -o chr17.png      # one chromosome
cnvkit.py scatter sample.cnr -s sample.cns -v sample.vcf.gz -o baf.png  # with BAF panel
cnvkit.py diagram sample.cnr -s sample.cns -o diagram.pdf             # ideogram
cnvkit.py heatmap cohort/*.cns -d -o cohort_heatmap.pdf              # cohort, desaturated

Passing -v with a VCF adds a B-allele-frequency panel — use it whenever LOH matters.

Genome-Wide log2 Profile with Segments

Goal: Render a publication genome-wide CNV profile with colored segments.

Approach: Map per-bin log2 to cumulative genomic coordinates, plot bins as faint points, overlay segment medians as colored horizontal lines, mark chromosome boundaries.

import pandas as pd
import matplotlib.pyplot as plt

def plot_genome_profile(cnr_file, cns_file, output=None, gain=0.3, loss=-0.3):
    '''Genome-wide log2 scatter with segment overlay.'''
    cnr = pd.read_csv(cnr_file, sep='\t')
    cns = pd.read_csv(cns_file, sep='\t')
    chroms = [f'chr{i}' for i in range(1, 23)] + ['chrX', 'chrY']

    offsets, cum = {}, 0
    for c in chroms:
        sub = cnr[cnr['chromosome'] == c]
        if sub.empty:
            continue
        offsets[c] = cum
        cum += sub['end'].max()

    cnr = cnr[cnr['chromosome'].isin(offsets)].copy()
    cnr['x'] = cnr.apply(lambda r: offsets[r['chromosome']] + r['start'], axis=1)

    fig, ax = plt.subplots(figsize=(16, 4))
    ax.scatter(cnr['x'], cnr['log2'], s=1, c='0.7', alpha=0.5, rasterized=True)
    for _, seg in cns.iterrows():
        if seg['chromosome'] not in offsets:
            continue
        x0 = offsets[seg['chromosome']] + seg['start']
        x1 = offsets[seg['chromosome']] + seg['end']
        color = 'red' if seg['log2'] > gain else 'blue' if seg['log2'] < loss else '0.3'
        ax.hlines(seg['log2'], x0, x1, colors=color, linewidth=2.5)
    for c, x in offsets.items():
        ax.axvline(x, color='0.9', linewidth=0.5)
    ax.axhline(0, color='black', linewidth=0.6)
    ax.set_ylim(-2, 2)
    ax.set_ylabel('log2 copy ratio')
    ax.set_xlabel('genomic position')
    fig.tight_layout()
    if output:
        fig.savefig(output, dpi=200)
    return fig, ax

Combined log2 + B-Allele-Frequency Panel

Goal: Show total copy number and allelic imbalance together so CN-neutral LOH and allele-specific gains are visible.

Approach: Stack two axes — log2 on top, BAF below. Mirror BAF about 0.5 so allelic imbalance reads as deviation from the center line.

import numpy as np

def plot_log2_baf(cnr_file, baf_df, output=None):
    '''Two-panel plot: log2 copy ratio above, B-allele frequency below.
    baf_df: columns chromosome, position, baf (germline-het sites only).'''
    cnr = pd.read_csv(cnr_file, sep='\t')
    fig, (ax_cn, ax_baf) = plt.subplots(2, 1, figsize=(16, 6), sharex=True)

    ax_cn.scatter(range(len(cnr)), cnr['log2'], s=1, c='0.6', alpha=0.5)
    ax_cn.axhline(0, color='black', linewidth=0.6)
    ax_cn.set_ylabel('log2 ratio')
    ax_cn.set_ylim(-2, 2)

    # Plot BAF and its mirror; a tight band at 0.5 = balanced, split bands = imbalance/LOH
    ax_baf.scatter(range(len(baf_df)), baf_df['baf'], s=2, c='0.4', alpha=0.5)
    ax_baf.scatter(range(len(baf_df)), 1 - baf_df['baf'], s=2, c='0.4', alpha=0.5)
    ax_baf.axhline(0.5, color='black', linewidth=0.6)
    ax_baf.set_ylabel('B-allele frequency')
    ax_baf.set_ylim(0, 1)
    ax_baf.set_xlabel('het SNP index')
    fig.tight_layout()
    if output:
        fig.savefig(output, dpi=200)
    return fig

A copy-neutral LOH region shows log2 ~ 0 but BAF splitting away from 0.5 — invisible on any log2-only plot.

Cohort Heatmap

Goal: Show recurrent CNV patterns across a cohort.

Approach: Resample every sample's segments onto a common genomic bin grid, stack into a samples-by-bins matrix, render with a diverging colormap centered at zero.

import seaborn as sns

def plot_cohort_heatmap(cns_files, bin_size=1_000_000, output=None):
    '''Recurrent-CNV heatmap across a cohort on a uniform bin grid.'''
    chroms = [f'chr{i}' for i in range(1, 23)]
    columns = []
    for c in chroms:
        columns += [(c, b) for b in range(0, 250_000_000, bin_size)]
    matrix = {}
    for f in cns_files:
        name = f.split('/')[-1].replace('.cns', '')
        cns = pd.read_csv(f, sep='\t')
        row = {}
        for c, b in columns:
            hits = cns[(cns['chromosome'] == c) &
                       (cns['start'] < b + bin_size) & (cns['end'] > b)]
            row[(c, b)] = hits['log2'].mean() if not hits.empty else 0.0
        matrix[name] = row
    df = pd.DataFrame(matrix).T
    fig, ax = plt.subplots(figsize=(14, max(4, 0.3 * len(cns_files))))
    sns.heatmap(df, cmap='RdBu_r', center=0, vmin=-1.5, vmax=1.5,
                xticklabels=False, ax=ax)
    ax.set_xlabel('genomic bin')
    ax.set_ylabel('sample')
    if output:
        fig.savefig(output, dpi=200, bbox_inches='tight')
    return fig

Caller-Native Diagnostic Plots

# GATK: denoised ratios + modeled segments with allelic info
gatk PlotModeledSegments --denoised-copy-ratios tumor.denoisedCR.tsv \
    --allelic-counts tumor.hets.tsv --segments tumor.modelFinal.seg \
    --sequence-dictionary reference.dict --output-prefix tumor -O plots/

ASCAT (ascat.runAscat ASPCF and sunrise plots), Sequenza (sequenza.results chromosome view and the cellularity/ploidy contour), and FACETS (plotSample) emit diagnostic plots — always inspect these to confirm the purity/ploidy fit before trusting downstream calls. They are diagnostic, not publication, figures.

Failure Modes

The diploid-baseline centering trap

Trigger: Plotting log2 from a hyper-aneuploid or whole-genome-doubled tumor with the y-axis centered on the data median or mode.

Mechanism: If most of the genome is at tetraploid baseline, centering on the mode places 4 copies at log2 0. Every true diploid region then appears deleted and the plot tells the opposite story.

Symptom: A genome-wide pattern of "loss" (or "gain") inconsistent with the BAF track or with the caller's ploidy estimate.

Fix: Anchor the y-axis baseline to the caller's ploidy estimate, not the data mode. Always show a BAF track alongside; if BAF says balanced where log2 says deleted, the centering is wrong.

log2 axis presented as if it were absolute copy number

Trigger: Labeling a log2 y-axis "copy number" or comparing log2 amplitudes across samples of different purity.

Mechanism: log2 ratio compresses with decreasing purity — a true CN=4 amplification at 40% purity has roughly half the log2 amplitude of the same event at 80% purity.

Symptom: A real amplification looks weaker than a passenger gain in a purer sample; cross-sample amplitude comparisons are meaningless.

Fix: For cross-sample or absolute claims, plot integer copy number from a purity-corrected caller, not raw log2. Label log2 axes "log2 copy ratio".

Cohort heatmap binning erases focal events

Trigger: Large uniform bins (e.g. 1-3 Mb) in a cohort heatmap.

Mechanism: A focal amplification (e.g. a few hundred kb at MYC or ERBB2) is averaged with flanking neutral sequence and disappears.

Symptom: Known recurrent focal drivers absent from the heatmap; only arm-level events visible.

Fix: Use a bin size matched to the question — Mb bins for arm-level surveys, gene-centric or GISTIC peak regions for focal drivers. Consider a separate gene-level panel.

Quantitative Thresholds

Common Errors

References

• Talevich E et al 2016. CNVkit: genome-wide copy number detection from targeted DNA sequencing. PLoS Comput Biol 12:e1004873
• Van Loo P et al 2010. Allele-specific copy number analysis of tumors. PNAS 107:16910 (BAF interpretation)
• Gel B, Serra E 2017. karyoploteR: an R/Bioconductor package to plot customizable genomes. Bioinformatics 33:3088

Related Skills

• copy-number/cnvkit-analysis - Generates the .cnr/.cns inputs and built-in plots
• copy-number/gatk-cnv - GATK denoised ratios and modeled-segment plots
• copy-number/allele-specific-copy-number - Source of BAF/MAF tracks and ploidy estimates
• copy-number/recurrent-cnv - Cohort-level recurrence underlying heatmaps
• data-visualization/ggplot2-fundamentals - General publication-figure grammar
• data-visualization/circos-plots - Circular genome layouts for CNV + SV