Question

I have a data that looks like this:

> print(dat)
    cutoff  tp    fp
1    0.6    414 45701
2    0.7    172 16820
3    0.8    51  4326
4    0.9    49  3727
5    1.0    0     0

I want to plot them in reverse-order from smallest dat$tp to largest.
However this code plot them in order like above (i.e. largest to smallest) instead.

> fp_max <- max(dat$fp);
> tp_max <- max(dat$tp);
> op <- par(xaxs = "i", yaxs = "i")
> plot(tp ~ fp, data = dat, xlim = c(0,fp_max),ylim = c(0,tp_max), type = "n")
> with(dat, lines(c(0, fp, fp_max), c(0, tp, tp_max),  lty=1, type = "l",  col = "black"))
> lines( par()$usr[1:2], par()$usr[3:4], col="red" )

How can I modify the code above to address the problem?

Of course, the x-axis & y-axis coordinates should be from smallest to largest value

The following shows the result of my current code.

Notice that the line started at 0,0 and it ‘goes back’ to 0 again.
we want to avoid it going back to 0.

Answer 1

Ahh, I understand.

It’s because lines draws lines between the points in the order they are given.

There are a few ways you could get around this:

1. do type='l' in your plot command and then with(dat,lines(...)) is not necessary:

   # can also do the col='black',lty=1 in here.
   plot(tp ~ fp, data = dat, xlim = c(0,fp_max),ylim = c(0,tp_max), type = "l")
   

   Note that by definition of your fp_max and tp_max, you will include the point (fp_max,tp_max) already. And as long as you have a row with (0,0) for tp and fp in dat, you’ll also get the (0,0) point.

2. Sort dat$tp and use that to sort dat$fp too:

   plot(tp ~ fp, ..., type='n')
   # sort dat$tp
   obj <- sort(dat$fp,index.return=T)
   # use obj$x as tp and obj$ix to sort dat$fp prior to plotting
   with(dat, 
        lines(c(0, obj$x, fp_max), c(0, tp[obj$ix], tp_max),  
              lty=1, type = "l",  col = "black"))