Spectrograms of two heating events from simulation A. Top row: bright blue grain from the pixel that is marked with a cyan cross in Fig. 6. Bottom row: bright red grain from the pixel that is marked with a magenta cross in Fig. 6. Both events display an excess of chromospheric heating, shown in the column to the right. The spectrograms are scaled non-linearly with a γ factor to enhance contrast. For Mg II h and Ca II H, γ = 1/4; for Ca II 8542 Å, γ = 1/2. The left inset in the spectrograms shows the cluster class of the spectra at a given time-step: blue and red colours correspond to blue and red grains; light blue and light red colours correspond to clusters where the Ca II lines show weaker emission.