Some embodiments provide a program that provides a graphical user interface (GUI). The GUI includes a display area for displaying an image that includes several pixels. Each pixel includes a set of color component values. The GUI includes a waveform monitor for displaying a graph that includes several graphical representations of the several pixels in the image. Each graphical representation is (1) plotted along a first axis of the graph based on a position of a corresponding pixel in the image and (2) plotted along a second axis of the graph based on the set of color component values of the corresponding pixel in the image. A color of each graphical representation is similar to a color of the corresponding pixel that is used for displaying the pixel in the display area.