add ticks to parcoord (parallel coordinates plot)

Question

The parcoord function from the MASS package looks quite ok, but how can I add ticks to the four y-axis?

Code is here:

 ir <- rbind(iris3[,,1], iris3[,,2], iris3[,,3])
 parcoord(log(ir)[, c(3, 4, 2, 1)], col = 1 + (0:149)%/%50)

Answer 1

Did you try setting var.label=T in the parcoord function? Is that insufficient?

Otherwise i didn't see any easy way to change the axis with the default function. But it turns out that function is pretty short and we can easily steal ideas form it and make our own version that can. Here's one way to modify it

parcoordlabel<-function (x, col = 1, lty = 1,  lblcol="blue",...) 
{
    df <- as.data.frame(x)
    pr <- lapply(df, pretty)
    rx <- lapply(pr, range, na.rm = TRUE)
    x <- mapply(function(x,r) {
            (x-r[1])/(r[2]-r[1])
        },
        df, rx)
    matplot(1L:ncol(x), t(x), type = "l", col = col, lty = lty, 
        xlab = "", ylab = "", axes = FALSE, ...)
    axis(1, at = 1L:ncol(x), labels = colnames(x))
    for (i in 1L:ncol(x)) {
        lines(c(i, i), c(0, 1), col = "grey70")
        text(c(i, i), seq(0,1,length.out=length(pr[[i]])), labels = pr[[i]], 
            xpd = NA, col=lblcol)
    }
    invisible()
}

and you can run it with

ir <- log(rbind(iris3[,,1], iris3[,,2], iris3[,,3]))[,c(3,4,2,1)]
parcoordlabel(ir, col = 1 + (0:149)%/%50)

Now i will admit it's a bit ugly. But hopefully you can understand the pieces well enough to customize how you like