PlantUML does almost exactly this for PNG and SVG, it embeds a (post-processed) copy of the input as an iTXt chunk or SGML comment respectively. It can be recov

PlantUML does almost exactly this for PNG and SVG, it embeds a (post-processed) copy of the input as an iTXt chunk or SGML comment respectively. It can be recovered from those output formats with plantuml's "-metadata" option. Note post-processed: if you use the "!include" extension (for sprites or other reasons) then the saved content won't exactly match the input, but for simple diagrams it's ideal.