This program is designed to allow you to model the neutral hydrogen line spectrum (a.k.a. the 21-cm line) as a function of galactic longitude of a model Milky Way galaxy as viewed from the Sun (assumed to be 8.5 kpc out from the center). In other words, you are modelling the $\ell$-v diagram of the Milky Way.
This interactive interface initially loads the observed rotation curve and neutral gas density profile for the Milky Way and generates a synthetic $\ell$-v diagram. You can interactively sketch a new rotation curve or gas density profile and the synthetic $\ell$-v diagram updates to reflect those changes. You can also modify the gas velocity dispersion by sliding the slider labeled $\sigma_v$ and see the effect that has.