Illustration of the EM-DM relation for two simple models, discussed in Sect. 2.3.1. The red graph shows a short LoS with relatively weak variations in n_e, while the blue one shows a long LoS with strong variations. Albeit depicting two completely different environments, the calculated DM, EM, and ${\text{L}}_{{\text{DM}}^2\text{/EM}}$ values for an observer sitting at L = 0 approximately agree. The units in this illustration are arbitrary, as the effect is scale-invariant. Its main intention is to demonstrate the interpretational limitations of Eq. (11). A more physical illustration of the Milky Way is provided later in Sect. 4.1.3.