The image_png() function supports options like -notrans which is needed to disable manually the transparency for PNG graphics. Without this option PNGs created by rrdtool caused some color problems in Acrobat Reader when watching the PDF. Cause of this was that with an alpha channel inside the PNG a DeviceGray colorspace is layed below the image. The alpha channel is always there but it is not used at all by rrdtool. Without this colorspace, Acrobat shows the PNGs correctly. I attached some patch to fix this for the image_png() function. The other image_xxx functions don't need it as they also don't use additional options like image_png().
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