A waveform is a data structure composed of time equidistant values. Basically, you could consider it as a cluster containing two scalar values : the start time, the time increment and an array : the signal values. And of course, a waveform graph is the graph constructed from a waveform, using the time information to construct the t (X) scale and the signal values as Y values.
An XY graph is a graph built from data which do not need to be time equidistant. The data are two arrays for the X and Y axis, combined into a cluster (you can also use an array of clusters of Xand Y values)
For instance, you could record temperature and pressure at fixed time intervals and present the results as two waveform graphs : T = f(time) and P = f(time). But to present the evolution of pressure against temperature, you will need an XY graph.
Chilly Charly (aka CC)