how to control ticks in plots with yasp or xasp

Hi,

A few times I tried to control the number and position of tick marks  
in plots with the yasp or xasp parameters. For example, a y axis was  
drawn by default with tick marks at 0, 20, 40, 80 and 100. I tried to  
get tick marks every 10 by adding

yasp=(0, 100, 10)

but this had no effect at all. I know I can draw the axis and tick  
marks manually, but often this simple option would suffice if I could  
understand how to make it work.

Thanks in advance,

Denis Chabot