[R] multiple plots on same x axis

[IAIN GALLAGHER]

Hi.

I have two vectors of gene expression for each of
several days. I want to plot both vectors on the same
plot for a visual representation of up versus down
regulation. I've tried using add=T but that doesn't
work. 

eg

>plot(Day, gene1)
>plot(Day, gene2, add=T)

Any help would be appreciated.

Iain