Ratio of measured and predicted GR apsidal motion rates as a function of the dimensionless reduced mass η. The black dashed line and grey shadow area correspond to the best fit of Eq. (8), assuming γ = β = 1, and its 1-σ uncertainty. We highlight the broken x-axis and the different scales. The colour code is chosen such that redder colours correspond to systems with the smaller relative error, and therefore are dominating the fit.