[R] How to break an axis?

[Bo Peng]

Dear list,

I need to plot four almost horizontal lines with y-values around
1,3,4, 400. If I plot them directly, the first three lines will be
indiscernible so I am thinking of breaking y-axis into two parts, one
with range (0,5), another (395,400). Is there an easy way to do this?

I can think of two ways: 
1. use two plots and draw axes manually. The plot margins, are however
difficult to adjust.
2. use one plot, adjust y-values of the lines and draw y-axis
manually. But, how would I break y-axis and add separation symbols
*on* yaxis? (By separation symbol, I mean something like
------//------

Many thanks in davance.
Bo