png to plt conversion represents export of PNG pictures to HP Graphics, typically in effort to create a file that can be printed physically on plotters and similar large-scale printers.
HP format is supported in most dedicated graphics tool as one of the export option, so it shouldn't be difficult to find one capable of such export. Just not every .png file picture is suitable for this task though.
The topic of file preparation (and exports) for plotters is well-covered on the Internet and some guides can be found on YouTube as well.
Updated: October 24, 2023