[R] Highlighting a few bars in a barplot

[Michael Bibo]

Daniel Brewer <daniel.brewer <at> icr.ac.uk> writes:

> 
> Hello,
> 
> I have a bar plot where I am already using colour to distinguish one set
> of samples from another.  I would also like to highlight a few of these
> bars as ones that should be looked at in detail.  I was thinking of
> using hatching, but I can't work out how or if you can have a background
> colour and hatching which is different between bars.  Any suggestions on
> how I should do this?
> 
> Thanks
> 
> Dan

Hi Dan,

The following code was part of a response to another person off-list recently. 
She wanted to use different hatching angles to highlight different bars which
were already black-and-white coded stacked bars.  This approach basically
superimposes hatched bars over the coloured bars, using base graphics. It uses
the "add=TRUE" argument to superimpose the plots.  The same principle should
work for what you want.

X1 <- c(2300,2110)
X2 <- c(1300,2220)
X3 <- c(1300,1100)
X4 <- c(450,650)
data <- cbind (X1,X2,X3,X4)
colnames(data) <- c("sample 1","sample 2","sample 3","sample 4")

par(las=1)
par(mar=c(5,5,4,2))

barplot(
  data,
  beside=FALSE,
  horiz=TRUE,
  col=c("black","white"),
  xlim = c(0,5000)
)
barplot(
  cbind(X1,NA,X3),      # NA is necessary to correctly space the bars
  names.arg=rep("",3),  # so that the labels are not overwritten
  xlim = c(0,5000),
  beside=FALSE,
  horiz=TRUE,
  density=8,            # how dark the cross-hatching lines are
  angle=0,              # angle of hatch lines
  col="black",          # colour of hatch lines
  add=TRUE       # superimposes cross-hatched bars over original bars
)
barplot(
  cbind(NA,X2,NA,X4),
  names.arg=rep("",4),   # a second superimposed plot
  xlim = c(0,5000),      # is necessary because of the
  beside=FALSE,          # limitations of the angle argument
  horiz=TRUE,
  density=8,
  angle=90,
  col="black",
  add=TRUE
) 



Hope it helps,

Michael Bibo
michael_bibo<at>health.qld.gov.au