How to easily visualize a matrix?

Question

When doing matrix operations, I would like to be able to see what the results of my calculations are, at least to get a rough idea of the nature of the matrices going in and coming out of the operation.

How can I plot a matrix of real numbers, so that the x axis represents columns, the y represents rows, and the color or size of a point represents the cell value?

Ultimately, I would like to display multiple plots, e.g. the right and left hand sides of an equation.

Here is some example code:

a <- matrix(rnorm(100), ncol = 10)
b <- diag(1,10)
c <- a*b

par(mfrow = c(1,3))
plot.matrix.fn <- function(m) {
   #enter answer to this question here
}
lapply(list(a,b,c), plot.matrix.fn)

update: since posting this question, I found that there are some great examples here: What techniques exists in R to visualize a "distance matrix"?

Answer 1

You could try something like (adjusting the parameters to your particular needs)

   image(t(m[nrow(m):1,] ), axes=FALSE, zlim=c(-4,4), col=rainbow(21))

producing something like

Answer 2

See ?image for a single plot (note that row 1 will be at the bottom) and ?rasterImage for adding 1 or more representations to an existing plot. You may want to do some scaling or other transformation on the matrix first.