DM variation rates versus census DMs. On both panels the upward/downward triangles indicate DM values increasing/decreasing with time. Unfilled triangles mark pulsars without significant DM variation rate (i. e. with | ΔDM/Δt | smaller than its uncertainty). For such pulsars the lower parts of the errorbars are not shown. Left: DM variation rates, obtained by comparing census DM measurements to the pulsar catalogue values. Lighter (green) marks show pulsars which have relatively large DM errors (>0.1 pc cm^-3), in either the census measurements or the pulsar catalogue. The dotted and dashed lines show unweighted and weighted linear fit to the data in log-log space, respectively, excluding the outlier in the top left corner. The relation of Hobbs et al. (2004b, grey line) is overplotted together with one order-of-magnitude scatter reported by the authors (grey shade). The outlier at DM ≈ 3 pc cm^-3, PSR J1503+2111 is discussed in Sect. 3.2. Right: rate of DM variations calculated by comparing census measurements to the recent low-frequency observations of Pilia et al. (2016, larger grey markers), Stovall et al. (2015, dark blue markers) and Zakharenko et al. (2013, light orange markers). See text for discussion.