[R] Hide line ends behind unfilled circles?

[Michael Kubovy]

Dear r-helpers,

xx <- c(0.000, 0.210, 0.714, 0.514, 1.000, 0.190, 0.590, 0.152)
yy <- c(0.000, 0.265, 0.256, 0.521, 0.538, 0.761, 0.821, 1.000)
aa <- c(19, 19, 19, 21, 19, 21, 21, 21)
x0 <- xx[c(1, 1, 2, 2, 2, 3, 3, 4, 4, 4, 5, 6, 6, 7)]
y0 <- yy[c(1, 1, 2, 2, 2, 3, 3, 4, 4, 4, 5, 6, 6, 7)]
x1 <- xx[c(2, 3, 3, 4, 6, 4, 5, 5, 6, 7, 7, 7, 8, 8)]
y1 <- yy[c(2, 3, 3, 4, 6, 4, 5, 5, 6, 7, 7, 7, 8, 8)]

plot(yy ~ xx, pch = aa, cex = 3)
segments(x0, y0, x1, y1)

Can anyone suggest a way of insuring that the lines are hidden behind  
the unfilled circles?
_____________________________
Professor Michael Kubovy
University of Virginia
Department of Psychology
USPS:     P.O.Box 400400    Charlottesville, VA 22904-4400
Parcels:    Room 102        Gilmer Hall
         McCormick Road    Charlottesville, VA 22903
Office:    B011    +1-434-982-4729
Lab:        B019    +1-434-982-4751
Fax:        +1-434-982-4766
WWW:    http://www.people.virginia.edu/~mk9y/