A Chart can be referenced at three points: height, width and depth (x, y and z in Cartesian space). To understand the differences among charts, you have to distinguish between dimensions in space and dimensions of data. The two dimensions are distinct.
Data can either be represented in two or in three dimensional space. In two dimensions, data is represented along the x and y axis. In three dimensions, data is projected back along the z axis.
Two dimensions of data must be represented in two dimensional space. At least three dimensions of data are necessary to use the third spatial dimension.
But three or more dimensions of data can be represented in two dimensional space. For example, cluster and stack represent data categories in the bar chart of two spatial dimensions (x and y axes only).
