Grayscale stacked area plot in R

I am plotting a stacked area plot in 开发者_StackOverflowR using the stackpoly() command in the plotrix package. The default colors are glaring, to say the least. Is there any way to use symbols or grayscale instead?

From ?stackpoly:

 col: Color to fill the polygons. If NULL, ‘rainbow’ will be called
      to generate the colors. If NA, the polygons will not be
      filled.

Slightly modifying the example in ?stackpoly:

stackpoly(matrix(cumsum(testx),nrow=10),main="Test Stackpoly I",
   xaxlab=c("One","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten"),
  border="black",staxx=TRUE,
col=gray(seq(0.1,0.9,length=10)))