Pair plot of best-fit α and E_peak values for the cut-off power law fit of the GRBs that are well constrained in our sample and those from the time-resolved Fеrтi/GBМ catalogue (with the BEST selection criterion fulfilled). For visual purpose we only include error bars for one typical time bin in the GBM and in the SPI analysis. The uncertainties on the parameters for the SPI data points are given in Table G.1 and those for the Fеrтi/GBМ data points can be found in the time-resolved GBM catalogue in Yu et al. (2016).