Plot margins for support vector machine

Question

i just set up a SVM in R with e1071. Unfortunately the plot of the margin and die hyperplane does not look as desired. I wanted the margin to pass through the support vectors. Shouldnt this be the case? Can anybody spot my mistake?

Here is my code:

rm(list=ls(all=TRUE))

x1s <- c(.5,1,1,2,3,3.5,     1,3.5,4,5,5.5,6)
x2s <- c(3.5,1,2.5,2,1,1.2,  5.8,3,4,5,4,1)
ys <- c(rep(+1,6),          rep(-1,6))
my.data <- data.frame(x1=x1s, x2=x2s, type=as.factor(ys))
my.data

library('e1071')
svm.model <- svm(type ~ ., data=my.data, type='C-classification', kernel='linear',scale=FALSE)

plot(my.data[,-3],col=(ys+3)/2, pch=19, xlim=c(-1,6), ylim=c(-1,6))
points(my.data[svm.model$index,c(1,2)],col="blue",cex=2) 

w <- t(svm.model$coefs) %*% svm.model$SV
b <- -svm.model$rho
p <- svm.model$SV

abline(a=-b/w[1,2], b=-w[1,1]/w[1,2], col="black", lty=1)
abline(a=--b/p[1,2], b=-w[1,1]/w[1,2], col="orange", lty=3)
abline(a=--b/p[3,2], b=-w[1,1]/w[1,2], col="orange", lty=3)

Answer 1

Your last 2 commands should be

abline(a=(-b-1)/w[1,2], b=-w[1,1]/w[1,2], col="orange", lty=3)
abline(a=(-b+1)/w[1,2], b=-w[1,1]/w[1,2], col="orange", lty=3)

Another way

plot(my.data[,-3],col=(ys+3)/2, pch=19, xlim=c(-1,6), ylim=c(-1,6))
points(my.data[svm.model$index,c(1,2)],col="blue",cex=2) 

x1min = min(x1s); x1max = max(x1s);
x2min = min(x2s); x2max = max(x2s);

coef1 = sum(svm.model$coefs*x1s[svm.model$index]);
coef2 = sum(svm.model$coefs*x2s[svm.model$index]);
lines(c(x1min,x1max),  (svm.model$rho-coef1*c(x1min, x1max))/coef2)
lines(c(x1min,x1max),  (svm.model$rho+1-coef1*c(x1min, x1max))/coef2, lty=2)
lines(c(x1min,x1max),  (svm.model$rho-1-coef1*c(x1min, x1max))/coef2, lty=2)