The experimental data in Fig. (resp. Fig. ) of the main text result from summation of 500 (resp. 1000) acquisitions with a 100 ms (resp. 50ms) exposure time. The spectral sampling of the data was 0.003 eV/channel, giving an effective energy resolution of ~9 meV, as measured by the FWHM of the zero-loss peak (this value differs from the 'nominal' resolution due to the slightly smaller spectral sampling used and longer acquisition times per frame, which provided higher signal per individual spectrum for slight broadening of the integrated zero-loss peak). Alignment of the spectra due to possible drift of the ZLP away from the 0 eV energy channel was performed using a centre-of-mass alignment algorithm, prior to integration of the final data. ZLP removal was carried out on the spectral sum, which was first normalised as intensity times energy loss (to account for scaling of the harmonic oscillator model with energy), using a power law model, fitted over an energy window 0.009 eV wide, immediately before the first loss feature .