R: Finding the intersect of two lines

Question

I have this simple code that creates 3 matrices and plots them:

Y=matrix(c(1,2,3,4), nrow=1)
X1=matrix(c(2,3,3.5,4.5))
X2=matrix(c(0.1, 0.2, 0.6, 1.1), nrow         


        

Answer 1

Because the two graphs use different x scales, this is a rather odd question. Getting the crossing point for the X2 line is easy, but the X1 line is a little more complicated.

## X2 line
AF2 = approxfun(X2, Y)
AF2(0.4)
[1] 2.5

The problem with the X1 line is that 0.4 on your graph means only X2=0.4, but X1 != 0.4. You can see that the 0.4 mark is half way between X1 = 2.5 and X1= 3, so we need to compute that value using X1 = 2.75.

AF1 = approxfun(X1, Y)
AF1(2.75)
[1] 1.75

Confirm with graph:

#Plotting
plot(X1, Y)+lines(X1,Y) + abline(v=0.4, col="red")
par(new=TRUE)
plot(X2, Y)+lines(X2,Y) 
abline(v=0.4)
points(c(0.4,0.4), c(1.75, 2.5), pch=20, col="red")

Answer 2

identify() can be used to locate points in a scatter plot by clicking with the mouse in the plot area. Hope this is what you're looking for. Check it out!