Flux–flux relation for NUV measurements from the literature and GALEX/NUV broad-band. The literature data comprise Mg iiλ 2800 Å line emission published by Mathioudakis & Doyle (1989, MD89; circles) and by France et al. (2013, FFL13; triangles) and broad-band emission in 1150-1790 Å excluding Lyα (FFL13). The Mg iiλ 2800 Å data points from MD89 are labelled with numbers corresponding to the following stars: 1 – Gl 83.1, 2 – Gl 411, 3 – Gl 412 A, 4 – Gl 803, 5 – Gl 832, 6 – Gl 876, 7 – Gl 908. Two of them (Gl 832 and Gl 876) have data from FFL13. All literature data for these two stars are connected with a vertical line. The dashed line denotes the 1:1 relation, the dash–dotted lines a factor 2 and the dotted lines a factor 5 difference between the two measurements.