Can you display other .png files? (There's one here.) Keep in mind nothing at all can be printed to stdout before the Content-type line.

lynx -mime_header -source -dump http://www.server.com/graph.cgi | less should look like:

HTTP/1.1 200 OK
Date: Wed, 11 Aug 2004 17:25:39 GMT
Server: Apache/1.3.31 (Unix) mod_fastcgi/2.4.2
Last-Modified: Sat, 05 Jun 2004 08:20:37 GMT
ETag: "2501b-13d6-40c18255"
Accept-Ranges: bytes
Content-Length: 5078
Connection: close
Content-Type: image/png

<89>PNG...gibberish...
[download]

Important items to note are 1) the complete lack of HTML anywhere in the result, 2) "Content-Type: image/png", 3) no blank lines at any point before "Content-Type: image/png", and 4) the text "<89>PNG" or similar immediately following the blank line followed by lots of gibberish. If your output is deviating from this, what do you see?