Missing option to make double-histograms larger or higher

Currently, when one plots with --eval, it results in a situation like this:

While this is nice if you want to squeeze the histograms into a single square block, there should an option to "enlarge" the canvas, so that the histograms are not "half-size" in the compression direction. The same goes if the histograms are stacked vertically instead.

Another option that would be cool would be to separate each histogram in a single page, using a multi-page PDF or various output files (if the format is PNG or JPEG). This would allow more plasticity when generating plots for papers.