A dense pixel display is a multivariate method of visualizing every single data value in a data set at the same time. Each data value is represented by the color of a single pixel. A dense pixel display contains subwindows that map another dimension, to allow for easy comparisons and identification of trends. The pixels are typically sorted based on another variable to cause similar values to be clustered, which makes comparisons easier.
Overview of DPDs with some examples (telecommunication data)