A Dataset is an object which contains all the information needed to specify a single set of data points. It is a very simple object which is really little more than a container for numeric sequences. It has fields for the common uses of a sequence, e.g.: x, y, z, yerr, etc. At its purest level, a Dataset is prepared by assigning a sequence of numbers to each relevant field (e.g., for a scatter plot, you'd assign one sequence to x and a corresponding sequence to y). However, methods such as setXY make typical cases easier.
Each Dataset is associated (by position in the Graph) with one PlotFormat object. So to make a graph with two different data formats (e.g., two lines, one in thick red and the other in dashed green), there would be two Datasets and two PlotFormats. A Dataset is sometimes called a ``data series'' in other graphing packages.
In addition to storing numeric data, a Dataset may also store a function which will be sampled at graph time to obtain the numbers to plot. This takes advantage of Python's functional programming capabilities, and is often useful for generating a quick graph of a function without manual sampling.