Gaia Data Release 2 - Observational Hertzsprung-Russell diagrams★ - Babusiaux, C; Van Leeuwen, F; Barstow. M. A; Jordi, C; Vallenari, A; Bossini, D; Souza, Ronaldo Eustáquio de; Teixeira, Ramachrisna (Astronomy & Astrophysics, 2018)