Thanks to GNUPlot.info for the retranscription of the help articles of gnuplot. It was a much handier database of information than just running gnuplot help for everything.
Also, thanks to This random 2009 blogpost that gave us the idea of simply turning an histogram to make an horizontal one. We didn't end up using their code, but credit is due where credit is due, we didn't realize that we could simply do this. In fact, we spent nearly a whole day tryin to figure out how to draw arbitrary boxes to make our own artificial horizontal histogram.
l.gp contains the main explaination of how plotting works, and d1.gp has most of the non-plotting parts described in it.