r - Plot a line chart with conditional colors depending on values

Question

I want to plot a line chart. Depending on values it should change its color. What I found is:

plot(sin(seq(from=1, to=10,by=0.1)),type="p", 
       col=ifelse(sin(seq(from=1, to=10,by=0.1))>0.5,"red","yellow"))

That works. But as soon as I change from type="p" to type="l" the conditional colouring disappears.

Is that behavior intended?

What is a solution with base graphics to plot a functional line with different colors?

Answer 1

Use segments instead of lines.

The segments function will only add to an existing plot. To create a blank plot with the correct axes and limits, first use plot with type="n" to draw "nothing".

x0 <- seq(1, 10, 0.1)
colour <- ifelse(sin(seq(from=1, to=10,by=0.1))>0.5,"red","blue")

plot(x0, sin(x0), type="n")
segments(x0=x0, y0=sin(x0), x1=x0+0.1, y1=sin(x0+0.1), col=colour)

See ?segments for more detail.