x0=c(2,2,3,4)
x1=x0
y0=c(2,3,2,2)
y1=c(4,3,4,4)
x=c(0,3)
y=x
plot(x,y,type='l',lty=0,xlim=c(0,5),ylim=c(0,5))
segments(x0,y0,x1,y1)

x=(0:100)*2*pi/100
y=sin(x)
plot(x,y,type='l',lty=1)
abline(h=0,lty=1,col='mediumblue')