R generate 2D histogram from raw data

Question

I have some raw data in 2D, x, y as given below. I want to generate a 2D histogram from the data. Typically, dividing the x,y values into bins of size 0.5, and count the number

Answer 1

The ggplot is elegant and fast and pretty, as usual. But if you want to use base graphics (image, contour, persp) and display your actual frequencies (instead of the smoothing 2D kernel), you have to first obtain the binnings yourself and create a matrix of frequencies. Here's some code (not necessarily elegant, but pretty robust) that does 2D binning and generates plots somewhat similar to the ones above:

    require(mvtnorm)
    xy <- rmvnorm(1000,c(5,10),sigma=rbind(c(3,-2),c(-2,3)))

    nbins <- 20
    x.bin <- seq(floor(min(xy[,1])), ceiling(max(xy[,1])), length=nbins)
    y.bin <- seq(floor(min(xy[,2])), ceiling(max(xy[,2])), length=nbins)

    freq <-  as.data.frame(table(findInterval(xy[,1], x.bin),findInterval(xy[,2], y.bin)))
    freq[,1] <- as.numeric(freq[,1])
    freq[,2] <- as.numeric(freq[,2])

    freq2D <- diag(nbins)*0
    freq2D[cbind(freq[,1], freq[,2])] <- freq[,3]

    par(mfrow=c(1,2))
    image(x.bin, y.bin, freq2D, col=topo.colors(max(freq2D)))
    contour(x.bin, y.bin, freq2D, add=TRUE, col=rgb(1,1,1,.7))

    palette(rainbow(max(freq2D)))
    cols <- (freq2D[-1,-1] + freq2D[-1,-(nbins-1)] + freq2D[-(nbins-1),-(nbins-1)] + freq2D[-(nbins-1),-1])/4
    persp(freq2D, col=cols)

For a really fun time, try making an interactive, zoomable, 3D surface:

require(rgl)
surface3d(x.bin,y.bin,freq2D/10, col="red")