Melting points are at standard pressure (1 atm) for most elements. Two elements never form a liquid at standard pressure: helium (requires ~25 atm to solidify) and arsenic (sublimes directly from solid to gas at 614°C — requires ~28 atm to melt). Both correctly show no liquid phase until pressure is raised on the pressure slider. Boiling points adjust with the pressure slider using an approximation of the Clausius-Clapeyron equation. Values for some synthetic/radioactive elements are estimated or unknown — the 14 unknown elements (z=104–118) have only ever been produced a few atoms at a time and decay too quickly for physical properties to be measured.
This tool was built for my own personal GCSE revision. You're welcome to use it, but it may contain errors and shouldn't be solely relied upon. Consider this a beta test. — Leon Dudhill