1. A method of controlling polishing, comprising: polishing a substrate; monitoring the substrate during polishing with an in-situ spectrographic monitoring system to generate a sequence of measured spectra; for each measured spectrum in the sequence of measured spectra, determining whether to include the measured spectrum as a selected spectrum based on at least one of determining the presence or absence of a feature in the measured spectrum, or determining a position of a feature in the measured spectrum relative to a prior measured spectrum from the sequence of measured spectra, such that less than all of the measured spectra are selected to generate a sequence of selected spectra; generating a sequence of values from the sequence of selected spectra; and determining at least one of a polishing endpoint or an adjustment for a polishing rate based on the sequence of values.