Okay, so usr is an argument of the par function (run the ?par command to find out more) which inputs a vector (x1, x2, y1, y2) and uses this vector to create the extremes of the plotting region. If I understand the documentation correctly (and I might not) if you used the vector (-10,10, -10, 10) it would give you the plot focused in on that region. Like this (not made in R because sometimes its easier to google for a picture):
