1. A non-transitory machine readable medium storing a program which when executed by at least one processing unit provides a graphical user interface (GUI), the GUI comprising: a display area for displaying an image comprising a plurality of pixels, each pixel comprising a set of color component values; and a waveform monitor for displaying a graph comprising a plurality of graphical representations of the plurality of pixels in the image, wherein 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, wherein 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.