An example using manipulate to modify a ggplot object. Requires RStudio.

manipulate example.R

aPlotFunction <- function(hh, ss, sz){
  zp1 <- qplot(data = cars, x = dist, y = speed,
               colour = I(hsv(hh/255, 1, 1)),
               shape = I(ss),
               size = I(sz))
  print(zp1 + theme_bw())
}

manipulate(
  aPlotFunction(hh, ss, sz),
  hh = slider(0, 255, step = 1, initial = 0),
  ss = slider(1, 255, step = 1, initial = 1),
  sz = slider(1, 20, step = 1, initial = 1))

Excellent!