Sometimes you will need to analyze data that are in the form of pairs, with one independent variable and one dependent variable in each pair. For example, the data pairs may be ages and weights of children, or hours studied and test scores of students.
The best way to represent these data graphically is with a scatterplot: plotting each independent variable as an x-coordinate, and each dependent variable as a y-coordinate. This allows the reader to quickly see if there is a relationship between the two variables and how strong the relationship is.
You may also analyze data that do not occur as pairs but as single numbers. Examples include the test scores of many students or the glucose levels of diabetic patients. These data are presented graphically with a frequency distribution.