plot(NULL, xlim=c(0,3), ylim=c(-10,10), xlab='t (s)', ylab='vy (m/s)',
     xaxs='i', main='velocità verticale (y) con ay=-g') 
grid()
abline(h=0)

p <- locator(1)    

vy0 = p$y[1]
g = 9.8

abline(vy0, -g, col='blue') 
abline(h=vy0,  col='blue', lty=2)

if (vy0 > 0) {
   t.vy.eq.0 = vy0 / g
   points(t.vy.eq.0, 0, col='blue')
   abline(h=-vy0,  col='blue', lty=2)
   points(2*t.vy.eq.0, -vy0, col='blue')
   abline(v=t.vy.eq.0, col='red', lty=3)
   abline(v=2*t.vy.eq.0, col='red', lty=3)
}