1. A method for generating a pixel-oriented graph, comprising: determining, by a computer, a first visual boundary for representing a first aggregate of a set of values of a variable of pixels depicted in the pixel-oriented graph, wherein the pixels correspond to respective data records, wherein the first aggregate of the set of values of the variable is computed by applying an aggregate function on the set of values of the variable, wherein the first visual boundary is selectable from among plural types of visual boundaries based on received input; after determining the first visual boundary, constructing, by the computer, a set of pixel blocks such that the pixel blocks are visually distinguished by the first visual boundary, each pixel block having a corresponding set of the pixels and each pixel having a pixel value that visually represents a corresponding one of the values of the variable.