Plotting a function with points()

Question

I am using R to plot a function, and want to add lines describing multiple functions to the same plot. To plot a function, I write:

plot(function(x){x}, 
     xlab="Celsius", xlim=c(-100, 100), 
     ylab="Degrees", ylim=c(-100, 100))

This would plot a 1:1 line. If I want to plot a different function on the same graph, I can use the points() function but this requires data values for x to be provided such that it plots length(x) points (joined by lines) as:

points(x=seq(-100, 100, by=0.1), 
       y=c(seq(-100, 100, by=0.1)-32)*5/9, 
       typ="l", col="red")

Is it possible to add lines to a plot when plotting a function rather than having to calculate data points using points() or another function? Essentially, it would be something like this:

plot(function(x){x}, 
     xlab="Celsius", xlim=c(-100, 100), 
     ylab="Degrees", ylim=c(-100, 100))
points(function(x){(x-32)*5/9}, 
       typ="l", col="red")

This is just an example, it shows the relationship between degrees Celsius on the X axis, and degrees on the Y axis in Celsius (black) and Fahrenheit (red). In reality I want to plot multiple complex functions but that would just add noise to the question.

One solution I found is

plot(function(x){x}, 
     xlab="Celsius", xlim=c(-100, 100), 
     ylab="Degrees", ylim=c(-100, 100))
par(new=TRUE)
plot(function(x){(x-32)*5/9}, 
     xlab="", xlim=c(-100, 100), 
     ylab="", ylim=c(-100, 100),
     axes=FALSE, col="red")

But it seems cumbersome having to define limits and labels and AXES=FALSE each time.

user_13460602 · Answer

You can use the plot function twice and add add = TRUE for the second plot.

With plot, you can also use from and to parameters to avoid repeating the y-axis limits, although it will keep the y-axis limits defined in the first plot (so it might not be optimal).

plot(function(x){x}, 
     xlab="Celsius", xlim=c(-100, 100), 
     ylab="Degrees", ylim=c(-100, 100))
plot(function(x) {(x-32)*5/9}, from = -100, to = 100, typ="l", col="red", add=T)

As mentioned by @Roland and @user2554330, you can also use curves if you want to plot multiple lines from the same function, and use () to avoid assigning the function beforehand, with add = i!=1 standing for add = T at every iteration except the first one.

for(y in 1:10) {
  curve((x + 10*y), from=-100, to=100, add=i!=1)
}

Plotting a function with points()

1 Answers