correlation

Dear r-sig-ecology group,

I am trying to find correlation between two variables. whereas they are non-normal used Spearman in this way:

 cor.test(SiO4,density,method="spearman")

Spearman's rank correlation rho
data:  SiO4 and density
S = 5263.445, p-value = 0.1184
alternative hypothesis: true rho is not equal to 0
sample estimates:
      rho
-0.296415

plot(SiO4, density)

but i cant draw the line and in others same files, the problem is same
and what is meaning this message:

library (graphics)

abline(density~SiO4)

Error in int_abline(a = a, b = b, h = h, v = v, untf = untf, ...) :
  plot.new has not been called yet



.

Many thanks in advance,
mahnaz