R Piecewise Function Plotting

Question

So I read an answer here and then went ahead to define a piecewise function using ifelse.

I have a piecewise function defined to illustrate the penalty dollar amount p of not insuring to value at different loss amounts of x dollars.

p <- function(x){
  return(ifelse(x <=300, x - x*(300/400) , 
      ifelse(x > 300 & x <= 400,  300 - x*(300/400),
      ifelse(x > 400, 0, 0))))}
plot(p(0:500),type="l")

As you can see the output is not really very smooth, my question:

1. any plot option to make it perfectly smooth (as it should be here)?
2. how to plot only within the x ranging from 200 to 420, a plot point every 10 loss dollar increment? I suppose this maybe something really simple that I am not aware 200:10:420 ??