of a sample can be visualized without loss of accuracy by marking for each element a stroke at the corresponding place on an X axis. The resulting figure is a one-dimensional scatter diagram. (Usually one prefers to construct a one-dimensional histogram).
If the elements of the sample are pairs (Xi, Yi), 
of two variables they are marked as points in an X,Y-plane. In the resulting two-dimensional scatter diagram a correlation between X and Y (viz. a sizeable correlation coefficient) can often be detected by mere visual inspection of the data.