[R] How to draw a big heatmap?

[Ning Ma]

Hi everyone. How can I draw a big heatmaps?

png("foo.png",10000,10000)
heatmap(x)
dev.off()

where x is a big matrix, say 200*200.

The code above generates a small heatmap in the middle of the png file
and leaves big margins. I expect it to take up more space so that the
labels are not overlapping.

Thanks in advance.