ProPlot author here: Due to my day job as a graduate student, certain feature additions may be delayed to the summer of 2020. In the meantime, if you are interested in contributing to ProPlot, please see the contribution guide. Any amount of help is welcome!
The reference axes number. The axwidth, axheight, and aspectsubplots and figure arguments are applied to this axes, and aspect
ratio is conserved for this axes in tight layout adjustment.