De visualization

Version Compatibility

Reference examples tested with: DESeq2 1.42+, edgeR 4.0+, ggplot2 3.5+, limma 3.58+, matplotlib 3.8+

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

• R: packageVersion('<pkg>') then ?function_name to verify parameters

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

DE Visualization

Create visualizations for differential expression analysis using DESeq2 and edgeR built-in plotting functions.

Scope

This skill covers DE-specific built-in functions:

• DESeq2: plotMA(), plotPCA(), plotDispEsts(), plotCounts()
• edgeR: plotMD(), plotBCV(), plotMDS()
• Sample distance heatmaps and p-value distributions

For custom ggplot2/matplotlib implementations of volcano and MA plots, see data-visualization/volcano-and-ma-plots; for PCA / UMAP / t-SNE / PHATE, see data-visualization/dimensionality-reduction-plots.

Required Libraries

library(DESeq2)
library(ggplot2)
library(pheatmap)
library(RColorBrewer)
library(ggrepel)  # For labeled points

Installation

install.packages(c('ggplot2', 'pheatmap', 'RColorBrewer', 'ggrepel'))
# Optional: Enhanced volcano plots
BiocManager::install('EnhancedVolcano')

MA Plot

Goal: Visualize the relationship between mean expression and log fold change to assess DE results.

Approach: Plot log fold change against mean normalized counts, highlighting significant genes.

"Make an MA plot of my DE results" → Plot mean expression vs. fold change with significant genes colored, using plotMA or ggplot2.

DESeq2 MA Plot

# Built-in MA plot
plotMA(res, ylim = c(-5, 5), main = 'MA Plot')

# With custom alpha
plotMA(res, alpha = 0.05, ylim = c(-5, 5))

# Highlight specific genes
plotMA(res, ylim = c(-5, 5))
with(subset(res, padj < 0.01 & abs(log2FoldChange) > 2),
     points(baseMean, log2FoldChange, col = 'red', pch = 20))

Custom ggplot2 MA Plot

res_df <- as.data.frame(res)
res_df$significant <- res_df$padj < 0.05 & !is.na(res_df$padj)

ggplot(res_df, aes(x = log10(baseMean), y = log2FoldChange, color = significant)) +
    geom_point(alpha = 0.5, size = 1) +
    scale_color_manual(values = c('grey60', 'red')) +
    geom_hline(yintercept = 0, linetype = 'dashed') +
    labs(x = 'log10(Mean Expression)', y = 'log2 Fold Change', title = 'MA Plot') +
    theme_bw() +
    theme(legend.position = 'bottom')

edgeR MA Plot

# Using plotMD (mean-difference plot)
plotMD(qlf, main = 'MD Plot')
abline(h = c(-1, 1), col = 'blue', lty = 2)

Volcano Plot

Goal: Display statistical significance against fold change magnitude to identify the most important DE genes.

Approach: Plot -log10(p-value) vs. log2 fold change with threshold lines and optional gene labels.

"Create a volcano plot of differentially expressed genes" → Scatter plot of fold change vs. significance with colored significance regions and labeled top hits.

Basic Volcano Plot

res_df <- as.data.frame(res)
res_df$significant <- res_df$padj < 0.05 & abs(res_df$log2FoldChange) > 1

ggplot(res_df, aes(x = log2FoldChange, y = -log10(pvalue), color = significant)) +
    geom_point(alpha = 0.5, size = 1) +
    scale_color_manual(values = c('grey60', 'red')) +
    geom_vline(xintercept = c(-1, 1), linetype = 'dashed', color = 'blue') +
    geom_hline(yintercept = -log10(0.05), linetype = 'dashed', color = 'blue') +
    labs(x = 'log2 Fold Change', y = '-log10(p-value)', title = 'Volcano Plot') +
    theme_bw()

Volcano with Gene Labels

res_df <- as.data.frame(res)
res_df$gene <- rownames(res_df)
res_df$significant <- res_df$padj < 0.05 & abs(res_df$log2FoldChange) > 1

# Label top genes
top_genes <- head(res_df[order(res_df$padj), ], 10)

ggplot(res_df, aes(x = log2FoldChange, y = -log10(pvalue))) +
    geom_point(aes(color = significant), alpha = 0.5, size = 1) +
    scale_color_manual(values = c('grey60', 'red')) +
    geom_text_repel(data = top_genes, aes(label = gene),
                    size = 3, max.overlaps = 20) +
    geom_vline(xintercept = c(-1, 1), linetype = 'dashed') +
    geom_hline(yintercept = -log10(0.05), linetype = 'dashed') +
    labs(x = 'log2 Fold Change', y = '-log10(p-value)') +
    theme_bw()

EnhancedVolcano

library(EnhancedVolcano)

EnhancedVolcano(res,
    lab = rownames(res),
    x = 'log2FoldChange',
    y = 'pvalue',
    pCutoff = 0.05,
    FCcutoff = 1,
    title = 'Differential Expression',
    subtitle = 'Treatment vs Control')

PCA Plot

Goal: Assess sample clustering and identify batch effects or outliers via dimensionality reduction.

Approach: Apply variance-stabilizing transformation then project samples onto principal components, coloring by experimental variables.

"Show me a PCA plot of my samples" → Perform PCA on transformed expression data and visualize sample separation by condition and batch.

DESeq2 PCA

# Variance stabilizing transformation first
vsd <- vst(dds, blind = FALSE)

# Basic PCA
plotPCA(vsd, intgroup = 'condition')

# With more options
plotPCA(vsd, intgroup = c('condition', 'batch'), ntop = 500)

Custom PCA with ggplot2

vsd <- vst(dds, blind = FALSE)
pca_data <- plotPCA(vsd, intgroup = c('condition', 'batch'), returnData = TRUE)
percentVar <- round(100 * attr(pca_data, 'percentVar'))

ggplot(pca_data, aes(x = PC1, y = PC2, color = condition, shape = batch)) +
    geom_point(size = 4) +
    xlab(paste0('PC1: ', percentVar[1], '% variance')) +
    ylab(paste0('PC2: ', percentVar[2], '% variance')) +
    ggtitle('PCA Plot') +
    theme_bw() +
    theme(legend.position = 'right')

edgeR PCA (via limma)

library(limma)
log_cpm <- cpm(y, log = TRUE)
plotMDS(log_cpm, col = as.numeric(group), pch = 16)
legend('topright', legend = levels(group), col = 1:nlevels(group), pch = 16)

Heatmaps

Goal: Visualize expression patterns of significant genes across samples to reveal clusters and condition effects.

Approach: Z-score normalize VST-transformed counts for significant genes and cluster with pheatmap, annotating by condition.

"Make a heatmap of the top differentially expressed genes" → Extract significant genes, z-score normalize, and create a clustered heatmap with sample annotations.

Top DE Genes Heatmap

library(pheatmap)

# Get top significant genes
sig_genes <- rownames(subset(res, padj < 0.01))

# Get normalized counts
vsd <- vst(dds, blind = FALSE)
mat <- assay(vsd)[sig_genes, ]

# Scale by row (z-score)
mat_scaled <- t(scale(t(mat)))

# Create annotation
annotation_col <- data.frame(
    condition = colData(dds)$condition,
    row.names = colnames(mat)
)

pheatmap(mat_scaled,
         annotation_col = annotation_col,
         show_rownames = FALSE,
         clustering_distance_rows = 'correlation',
         clustering_distance_cols = 'correlation',
         color = colorRampPalette(c('blue', 'white', 'red'))(100),
         main = 'Top DE Genes')

Sample Distance Heatmap

vsd <- vst(dds, blind = FALSE)

# Calculate sample distances
sampleDists <- dist(t(assay(vsd)))
sampleDistMatrix <- as.matrix(sampleDists)

# Annotation
annotation <- data.frame(
    condition = colData(dds)$condition,
    row.names = colnames(dds)
)

pheatmap(sampleDistMatrix,
         annotation_col = annotation,
         annotation_row = annotation,
         clustering_distance_rows = sampleDists,
         clustering_distance_cols = sampleDists,
         color = colorRampPalette(c('white', 'steelblue'))(100),
         main = 'Sample Distance Matrix')

Gene Expression Heatmap

# Select genes of interest
genes_of_interest <- c('gene1', 'gene2', 'gene3', 'gene4', 'gene5')
mat <- assay(vsd)[genes_of_interest, ]

pheatmap(mat,
         scale = 'row',
         annotation_col = annotation_col,
         show_rownames = TRUE,
         cluster_cols = TRUE,
         cluster_rows = TRUE,
         main = 'Genes of Interest')

Dispersion Plot

Goal: Assess the fit of the dispersion model to verify DE analysis assumptions.

Approach: Plot gene-wise, fitted, and final dispersion estimates against mean expression.

DESeq2

plotDispEsts(dds, main = 'Dispersion Estimates')

edgeR

plotBCV(y, main = 'Biological Coefficient of Variation')

Counts Plot for Individual Genes

Goal: Visualize expression of a specific gene across samples and conditions.

Approach: Extract per-sample counts for a gene and plot by condition using plotCounts or ggplot2.

DESeq2

# Plot counts for a specific gene
plotCounts(dds, gene = 'GENE_NAME', intgroup = 'condition')

# With ggplot2
d <- plotCounts(dds, gene = 'GENE_NAME', intgroup = 'condition', returnData = TRUE)
ggplot(d, aes(x = condition, y = count, color = condition)) +
    geom_point(position = position_jitter(width = 0.1), size = 3) +
    scale_y_log10() +
    ggtitle('GENE_NAME Expression') +
    theme_bw()

edgeR

# Get CPM for a gene
gene_idx <- which(rownames(y) == 'GENE_NAME')
cpm_gene <- cpm(y)[gene_idx, ]

# Plot
df <- data.frame(cpm = cpm_gene, group = group)
ggplot(df, aes(x = group, y = cpm, color = group)) +
    geom_point(position = position_jitter(width = 0.1), size = 3) +
    scale_y_log10() +
    labs(y = 'CPM', title = 'GENE_NAME Expression') +
    theme_bw()

P-value Histogram

Goal: Diagnose the quality of the DE analysis by examining the raw p-value distribution.

Approach: Histogram of raw p-values; a uniform distribution with a peak near zero indicates a well-calibrated test.

# Check p-value distribution (should be uniform under null with peak near 0)
res_df <- as.data.frame(res)
ggplot(res_df, aes(x = pvalue)) +
    geom_histogram(bins = 50, fill = 'steelblue', color = 'white') +
    labs(x = 'P-value', y = 'Frequency', title = 'P-value Distribution') +
    theme_bw()

Saving Plots

Goal: Export publication-quality plots in vector or raster formats.

Approach: Use pdf/png devices or ggsave with appropriate resolution and dimensions.

# Save as PDF (vector)
pdf('volcano_plot.pdf', width = 8, height = 6)
# ... plot code ...
dev.off()

# Save as PNG (raster)
png('volcano_plot.png', width = 800, height = 600, res = 150)
# ... plot code ...
dev.off()

# Using ggsave for ggplot objects
p <- ggplot(...) + ...
ggsave('plot.pdf', p, width = 8, height = 6)
ggsave('plot.png', p, width = 8, height = 6, dpi = 300)

Color Palettes

Goal: Select appropriate color schemes for heatmaps and categorical data.

Approach: Use RColorBrewer palettes -- diverging for expression, sequential for distances, qualitative for groups.

# For heatmaps
library(RColorBrewer)

# Diverging (for expression: blue-white-red)
colorRampPalette(rev(brewer.pal(n = 7, name = 'RdBu')))(100)

# Sequential (for distances)
colorRampPalette(brewer.pal(n = 9, name = 'Blues'))(100)

# For categorical groups
brewer.pal(n = 8, name = 'Set1')

Quick Reference: Common Plots

Plot	Purpose	Function
MA plot	LFC vs mean expression	plotMA(), plotMD()
Volcano	LFC vs significance	ggplot2, EnhancedVolcano
PCA	Sample clustering	plotPCA(), plotMDS()
Heatmap	Gene patterns	pheatmap()
Dispersion	Model fit	plotDispEsts(), plotBCV()
Counts	Individual genes	plotCounts()

Diagnostic Interpretation

P-value Histogram

Check raw p-value distribution before trusting DE results:

Shape	Meaning	Action
Uniform + spike near 0	Correct: null genes uniform, true DE near 0	Proceed normally
Anti-conservative (U-shape, spikes at 0 and 1)	Inflated significance; unmodeled batch or violated assumptions	Check for batch effects, verify model specification
Conservative (depleted near 0, spike near 1)	Over-correction; too many covariates or wrong dispersion	Simplify model, check dispersion plot
Spike at p = 1 only	Discrete artifact from low-count genes	Pre-filter more aggressively

MA Plot Diagnostics

Pattern	Meaning
Symmetric cloud centered at LFC = 0	Correct normalization
Cloud shifted up or down	Normalization failure; majority-DE experiment may violate assumptions
Funnel shape widening at low expression	Expected — low-count genes have noisier fold changes
Discrete horizontal bands	Low-count artifacts; consider stronger pre-filtering

Volcano Plot: Shrunken LFCs

Use shrunken LFCs (apeglm/ashr) on the x-axis and un-shrunken p-values on the y-axis. This combination gives stable fold change estimates while preserving the original significance assessment. Without shrinkage, low-count genes with extreme but unreliable fold changes dominate the plot edges.

Dispersion Plot Diagnostics

Pattern	Meaning
Gene-wise points scattered around fitted line	Good model fit
Gene-wise points far above fitted line	Possible outlier genes or unmodeled batch effects
Fitted line flat (no trend)	Unusual — check if data is over-filtered or has unusual structure
Final estimates much lower than gene-wise	Expected — shrinkage toward the fitted trend

PCA Diagnostics

Pattern	Meaning	Action
Clear separation by condition on PC1/PC2	Strong biological signal	Proceed
Separation by batch, not condition	Batch effect dominates	Include batch in model; do NOT use corrected counts for DE
One sample far from its group	Potential outlier or sample swap	Check library QC metrics; consider removing
No separation on PC1/PC2 but present on PC3+	Subtle effects	May still find DE genes; check dispersion estimates

Related Skills

• deseq2-basics - Generate DESeq2 results for visualization
• edger-basics - Generate edgeR results for visualization
• de-results - Filter genes before visualization
• data-visualization/volcano-and-ma-plots - Custom ggplot2/matplotlib volcano + MA with LFC shrinkage
• data-visualization/dimensionality-reduction-plots - PCA / UMAP / t-SNE / PHATE
• data-visualization/heatmaps-clustering - Advanced heatmap customization