[R] Grid Points

[Anupam Tyagi]

How do I put grid points (not grid lines) as the base layer of an xyplot? 

Is there a way to vary the interval at which x and y grid points are placed?

Is it possible to start a graph so that Y axis begins at 500 and ends at 800? I
am only interested in focusing on the relative distance between the points whose
values are between 500 and 800, but not their relative distance from zero.

Is there a way in R to draw two graphs so that that "share" the same X axis, but
without the gap that mfrow() creates? The origin of the top graph should be
where the Y-axis of the bottom graph ends. It will be useful for what I am
trying to do to have the X axis of the top graph be invisible.

Anupam.