Visualization of the data space in the case of only two Faraday rotation measurements. Each coordinate denotes the possible values for the two measurements d₁ and d₂ at two sky positions. The data vector, d = (d_1,d₂)^† is the sum of a vector of Galactic Faraday depth, φ_g = (φ_g,1,φ_g,2)^†, of a vector of extragalactic Faraday depth, φ_e = (φ_e,1,φ_e,2)^†, and of a measurement noise vector, n = (n₁,n₂)^†. The reconstructions of these three components, , , and , are shown as dashed arrows. Their sum is also equal to the data, however they differ from the correct components due to the impossibility to uniquely separate one data vector into three statistically independent components.