Relative accuracy of the emulator for predicting the persistent Betti functions of a KV450-like (top) and a Euclid-like (bottom) survey. The thin-coloured lines represent the ratio between predicted and measured values for the 26 different cosmologies, estimated from a cross-validation test. The x-axis lists the points at which the functions are evaluated (see Table B.1 for their numerical values). The thick black line corresponds to the mean over all lines. The grey shaded area corresponds to the 1σ standard deviation of the covariance matrix extracted from SLICS.