Flux-flux relationships between calcium lines (Ca ii H & K, left panel), and between Hα and Ca ii K (right panel). M dwarfs from this work are plotted with red filled squares; FGK stars from López-Santiago et al. (2010), Martínez-Arnáiz et al. (2010, 2011) with open circles, late-K and M stars from the literature (same references as for FGK stars) are shown by purple open squares; green stars denote the M0-M3 pre-MS M stars from Stelzer et al. (2013a). Possible young disc stars in our M star sample are shown by circles. The two stars discussed in Sect. 6.5 are indicated by diamonds. The black dash-dotted line represents our best fit; the relations for the “active” and “inactive” branches by Martínez-Arnáiz et al. (2011) are shown in light grey solid and dashed dark grey lines, respectively.