The dots represent actual tube plate characteristics, see "source" of this page, applet params Vp, Ip, Vg. Curves are produced by tube model with parameters Mu, Kg1, etc entered as applet params and adjustable by the sliders at the bottom. Your goal here is to align the curves with the dots as closely as possible, after which the best model parameters are FOUND!
Author: Dmitry Nizhegorodov (email@example.com). My other projects and articles