mhtplot: Manhattan plot

In gap: Genetic Analysis Package

Manhattan plot

Description

Draw a Manhattan plot for genome-wide association studies (GWAS) or any genome-indexed numeric score.

Usage

mhtplot(data, control = mht.control(), hcontrol = hmht.control(), ...)

Arguments

data	A matrix or data frame with at least three columns. The first three columns are interpreted as: chromosome position value (typically p-value) Column names are ignored. Factors are automatically converted.
control	A list produced by mht.control() controlling plot behaviour.
hcontrol	A list produced by hmht.control() defining highlighted markers or regions and labels.
...	Additional graphical arguments passed to graphics::plot() (e.g. pch, bg, xlab, ylab, ylim). If mar is supplied, the default margins are not modified.

Details

Produces a Manhattan plot in which genomic markers are arranged along the x-axis by chromosome and position.

By default the y-axis shows -log10(p), the standard GWAS significance scale. Set logscale = FALSE in mht.control() to plot raw values instead.

Rows containing missing values are removed automatically before plotting.

Chromosome handling

Chromosomes may be numeric or character. The prefix "chr" is removed automatically. Chromosomes are ordered numerically first, followed by character chromosomes (e.g. X, Y, MT).

X-axis spacing

• usepos = FALSE (default): markers are spaced evenly within chromosomes.

• usepos = TRUE: real chromosomal positions are used.

• gap inserts spacing between chromosomes when using real positions.

Colours

Chromosome colours alternate by default. Custom colours can be supplied via colors in mht.control(); colours are recycled as needed.

Axis tuning

Axis appearance can be controlled using:

• axis.cex — tick label size

• axis.lwd — axis and tick thickness

• axis.tck — tick length and direction

These settings are particularly useful when exporting high-resolution figures.

Horizontal thresholds

Horizontal reference lines can be drawn using the cutoffs parameter.

Highlighting regions

Highlighted regions are defined using hmht.control(). Matching markers are recoloured, labelled, and optionally boxed. Matching is performed by chromosome and position range.

Value

invisibly returns NULL.

Mathematical background

A Manhattan plot visualises genome-wide association statistics by mapping genetic markers to a two-dimensional coordinate system.

Y-axis transformation

P-values are transformed to improve visibility of small values:

y = -\log_{10}(p)

Small p-values therefore appear as large positive values and form the characteristic peaks used to identify strong associations.

Genome linearisation

Markers originate from multiple chromosomes and must be mapped onto a single continuous axis. For chromosome c:

x_i = pos_i + offset_c

where the chromosome offset is

offset_c = \sum_{k < c} \max(pos_k)

This produces a continuous genome coordinate system in which chromosomes appear sequentially without overlap.

Chromosome label placement

Chromosome labels are positioned at the midpoint of each chromosome:

mid_c = (\min(x_c) + \max(x_c)) / 2

This centres labels regardless of chromosome length.

Plot limits

The plotting region is defined using axis tick locations so that ticks and plotting limits coincide exactly.

Conceptually, a Manhattan plot is defined by two transformations:

x = \text{linearised genome coordinate}

y = -\log_{10}(p)

All other elements (colouring, thresholds, highlighting) are visual annotations applied to these transformed coordinates.

Author(s)

Jing Hua Zhao

See Also

mht.control(), hmht.control()

Examples

## -----------------------------------------------------------
## 1. Minimal example
## -----------------------------------------------------------
test <- matrix(c(
  1,1,4,
  1,1,6,
  1,10,3,
  2,1,5,
  2,2,6,
  2,4,8),
  byrow=TRUE, ncol=3)
mhtplot(test)
## Raw values instead of -log10
mhtplot(test, mht.control(logscale = FALSE))

## -----------------------------------------------------------
## 2. Simulated GWAS dataset
## -----------------------------------------------------------
set.seed(1)
affy <- c(40220,41400,33801,32334,32056,31470,25835,27457,22864,
          28501,26273,24954,19188,15721,14356,15309,11281,14881,
          6399,12400,7125,6207)
n.markers <- sum(affy)
n.chr <- length(affy)
gwas <- data.frame(
  chr = rep(1:n.chr, affy),
  pos = 1:n.markers,
  p   = runif(n.markers)
)
mhtplot(gwas)

## -----------------------------------------------------------
## 3. Publication-style figure
## -----------------------------------------------------------
ops <- mht.control(
  axis.cex = 1.4,
  axis.lwd = 2,
  axis.tck = -0.03
)
mhtplot(gwas, control = ops, pch = 19)

## -----------------------------------------------------------
## 4. Real positions + chromosome gaps
## -----------------------------------------------------------
ops2 <- mht.control(usepos = TRUE, gap = 10000)
mhtplot(gwas, control = ops2, pch = 19)

## -----------------------------------------------------------
## 5. Genome-wide significance thresholds
## -----------------------------------------------------------
ops3 <- mht.control(cutoffs = c(5, 7.3))
mhtplot(gwas, control = ops3, pch = 19)

## -----------------------------------------------------------
## 6. Highlight selected genes
## -----------------------------------------------------------
hdata <- data.frame(
  chr  = c(1,3,5),
  pos  = c(10000,50000,90000),
  p    = c(1e-8,1e-7,1e-9),
  gene = c("GENE1","GENE2","GENE3")
)
hops <- hmht.control(
  data = hdata,
  colors = "red",
  boxed = TRUE
)
mhtplot(gwas, control = ops, hcontrol = hops, pch = 19)
## A real study (data from gap.datasets package)
if (requireNamespace("gap.datasets", quietly = TRUE)) {
  data("mhtdata", package = "gap.datasets")

  data <- with(mhtdata, cbind(chr, pos, p))
  glist <- c("IRS1","SPRY2","FTO","GRIK3","SNED1","HTR1A","MARCH3",
             "WISP3","PPP1R3B","RP1L1","FDFT1","SLC39A14","GFRA1","MC4R")
  hdata <- subset(mhtdata, gene %in% glist)[c("chr","pos","p","gene")]

  ops  <- mht.control(colors = rep(c("lightgray","gray"),11),
                      labels = paste("chr",1:22,sep=""),
                      yline = 1.5, xline = 3)
  hops <- hmht.control(data = hdata, colors = "red", boxed = TRUE)

  mhtplot(data, ops, hops, pch = 19)
  title("Manhattan plot with genes highlighted")
}

## -----------------------------------------------------------
## 7. Export high-resolution PNG
## -----------------------------------------------------------
f <- tempfile(fileext = ".png")
png(f, height = 3600, width = 6000, res = 600)
opar <- par()
par(cex = 0.7)
mhtplot(gwas, control = ops, pch = 19)
par(opar)
dev.off()

## -----------------------------------------------------------
## 8. Miamiplot (see also vignette)
## -----------------------------------------------------------
if (requireNamespace("gap.datasets", quietly = TRUE)) {
  data("mhtdata", package = "gap.datasets")
  mhtdata <- within(mhtdata,{pr=p})
  miamiplot(mhtdata,chr="chr",bp="pos",p="p",pr="pr",snp="rsn")
}

gap documentation built on May 28, 2026, 9:07 a.m.