The left panel compares the v_d from previous works to our measurements. Here, “L” indicates Levin et al. (2016), “T” refers to Turner et al. (2021), “11 cm” compares against measurements we made in the 2-GHz range; “21 cm” refers to measurements we made in the 1-GHz range – in both cases the previously published measurements were rescaled to our observing frequency, as given in Col. (3) of Table 2. Data from Levin et al. (2016) were at 1500 MHz while Turner et al. (2021) used data at 1500 MHz as well as 820 MHz, but we only compare to the former. The three ellipses indicate groups of points that show significant offsets, which are discussed in more detail in the text. The right-hand panel shows violin plot for the distribution of the differences (Delta) of the four groups between the v_d of our measurements and the v_d from previous studies.