PNG generation error

[eluser]Unknown[/eluser]
Hi,
I'm using CodeIgniter to develop small webapp. In ordeto to generate charts I use jpgraph. There's no problem with using it stand alone. However, when I put the same algorithm into CodeIgniter, firefox says it can't render the picture. Whe I took a look in packets from server, I saw malformed PNG with unlikely long length. As jpgraph works perfectly without CodeIgniter on the same server, I assume that there must be sth wrong with CI. Any ideas?