Comparison of Δμ/μ measurement in this work and those in the literature. All measurements at 2.0 < z < 3.1 are based on the analysis of H₂ absorption. The filled larger blue star shows our result and the smaller red star shows the result from Rahmani et al. (2013). The downwards empty and filled triangles are the Δμ/μ measurements from van Weerdenburg et al. (2011) and Malec et al. (2010). The filled upward triangle and the empty and filled squares are, respectively, from King et al. (2008; 2011), and Wendt & Molaro (2012). The solid box and the open circle present the constraint obtained, respectively, by Rahmani et al. (2012) and Srianand et al. (2010) based on the comparison between 21-cm and metal lines in Mg ii absorbers under the assumption that α and g_p have not varied. The Δμ/μ at z < 1 are based on ammonia and methanol inversion transitions: their 5σ errors are shown. The two measurements at z ~ 0.89 with larger and smaller errors are, respectively, from Henkel et al. (2009) and Bagdonaite et al. (2013) based on the same system. The two Δμ/μ at z ~ 0.684 with larger and smaller errors are, respectively, from Murphy et al. (2008) and Kanekar (2011) based on the same system.