Left panel: histogram of the number of pixels as a function of the maximum value in their polarization signals (normalized to the average continuum intensity). The vertical solid-black line represents the 3σ-level. Right panel: histogram of pixels with polarization signals above a certain S/N. This last histogram is cumulative, while the first one is not. In both panels the color lines indicate the same: red (map A at Λ = 0°), green (map B at Λ = 30°), blue (map C at Λ = 40°). The solid lines refer to the linear polarization profiles (Stokes Q and U) while the dashed lines correspond to the circular polarization profiles (Stokes V). For comparison we also display, in solid and dashed-black curves, the expected behavior due to noise.