I saved a NarraFirma graph to a PNG file, but the file is empty.

Yes. This is a limitation of the library we are using to render a SVG graph to the PNG format. It happens (at least we’ve seen it) when you are using Firefox as your browser and your image contains texts (possibly popup texts) that contain special characters (umlauts, etc) that do not render correctly in the XML format. There is an easy fix: do the same thing using Chrome or Safari. This seems to be a Firefox-specific problem.