Charts - empty image files

[Ben]

Hi,
I have been using bugzilla 2.17.4 for a while, and the charts have always worked.  However, after moving the installation to a new box, I am unable to see any charts.  It turns out the gif files being created are just empty files.  I have also upgraded to 2.17.6 and have experienced the same problem here.  I don't think it is a permission issue because the files are being created, just not having anything written into them.  Could it be some issue with the GD library on the new box, or is there something else that I'm missing?
Thanks,
Ben