x1 <- -30; x2 <- 30; y1 <- -30; y2 <- 30; avvio <- 1
par( mai = c(0.5,0.5,0.1,0.1) )
while(1==1) {
plot(c(x1,x2),c(y1,y2),type="n",xlab="", ylab="", asp=1)
abline(h=seq(y1,y2,5),v=seq(x1,x2,5),lty=3,col="orange")
abline(h=0,v=0,col="blue",lty=2)
if (avvio==1) {text(0,-25.5,"clicca pił volte; infine premi ESC",col="red"); avvio <-0}
x <- c(13,11,10, 9, 7,7.5, 7, 9,10,11,13,17,20,20.5,18.5,21,20,17,13)
y <- c(17,16,15,13,10, 13,16,13,11,10, 9, 9,11,  12,  13,13,15,17,17)
x <- x-7; y <- y-10; lines(x,y)
p <- locator(1); p <- c(round(p$x*2)/2,round(p$y*2)/2);
polygon(c(-29,-21,-21,-29),c(21,21,29,29),col="white",border="white")
text(-25,27.5,p[1]); text(-25,22.5,p[2])
lines(x + p[1], y + p[2], col="red"); p <- locator(1)  }