Finding the number of clusters, k, to optimally cluster the Hβ spectra. Variation in the inertia (σ_k) with respect to k is shown in black. The presented σ_k is normalized with the total number of data points used in the training of the k-means model. The running difference σ_k + 1 − σ_k is plotted in blue. The vertical dotted black line indicates the used number of clusters, k = 100, for the final clustering.