1. A method of polishing, comprising: polishing a substrate; receiving an identification of a selected spectral feature to monitor during polishing; measuring a sequence of spectra of light reflected from the substrate while the substrate is being polished, at least some of the spectra of the sequence differing due to material being removed during the polishing; determining a location value and an associated intensity value of the selected spectral feature for each of the spectra in the sequence of spectra to generate a sequence of coordinates, the coordinates being pairs of location values and associated intensity values; and determining at least one of a polishing endpoint or an adjustment for a polishing rate based on the sequence of coordinates.