Line functions in R

Question

Hi, I was wondering if it was possible to graph three lines in R using fuctions. For instance, how could I get the functions:

3x+1 
4x+2
x+1

to show up on the same graph in r? Thanks so much!

Answer 1

First decide the bounds, say 0 to 100, and make an empty plot including those points:

plot(c(0,100), c(0,100))

possibly of course with optional parameters such as axes=, xlab=, ylab=, and so on, to control various details of the axes and titling/labeling; then, add each line with abline(a, b) where b is the slope and a is the intercept, so, in your examples:

abline(1, 3)
abline(2, 4)
abline(1, 1)

Of course there are many more details you can control such as color (col= optional parameter), line type (lty=) and width (lwd=), etc, but this is the gist of it.

Answer 2

Here's another way using matplot:

> x <- 0:10
> matplot(cbind(x, x, x), cbind(3*x+1, 4*x+2, x+1), 
          type='l', xlab='x', ylab='y')

matplot(X, Y, ...) takes two matrix arguments. Each column of X is plotted against each column of Y.

In our case, X is a 11 x 3 matrix with each column a sequence of 0 to 10 (our x-values for each line). Y is a 11 x 3 matrix with each column computed off the x vector (per your line equations).

xlab and ylab just label the x and y axes. The type='l' specifies that lines are to be drawn (see other options by typing ?matplot or ?plot at the R prompt).

One nice thing about matplot is that the defaults can be nice for plotting multiple lines -- it chooses different colors and styles per line. These can also be modified: see ?matplot (and lty for more detail).

Answer 3

You can also use the curve function. For example:

curve(3*x+1, from=-5, to=5)
curve(4*x+2, add=T)
curve(x+1, add=T)

Here the add parameter causes the plots to be put on the same graph