Server Disk Usage Report Graphs Don't Display Correctly.

I have set it up so that myself and several other members of the tech. department at our company get emailed daily with disk usage reports. Each drive on each server is reported, and a graph appears showing simply and graphically how many is being used. Its very nice. You guys did a great job with this. The problem however is that some don't have green for the low usage or red for the high usage - in fact they only have the black outline and the white box, with no white or red at all. This makes it difficult to tell very quickly how much of the drive is used. Why is this happening?
Attached is a picture of the PDF report graphs, including some with nothing in them.[/img]