Btw, I had submitted some time ago an RFE to be able to save chart in high resolution and was in the need to modify the PNGEncoder to specify the dpi resolution of the image.
The changed file can be found at this RFE : [ 777515 ] High resolution images saving
Btw, I forwarded the changes to the author of PNGEncoder, J. David Eisenberg, who answered me :
Thanks; glad you liked the package. I'm not doing much work with it,
since Sun is now shipping a library that does similar things.
So wel'll perhaps need to consider to change the package for the future...