Fig. 2
From: Spectral methods for imputation of missing air quality data
Five runs averaged max difference as a function of number of omitted data samples reconstructing the signal with the following imputation methods NN (blue), Linear Interpolation (green), substituting missing data with the Mean (cyan), EM (purple), DCT (yellow) and K-SVD (black). The color codes are detailed in a panel. The missing data was omitted either in random (a) or in batches (b). a NO2 (random locations). b SO2 (continuous batch)