The axis of a graph is basically a scale which function is to plot data, that is, the each data is identified by a variable which is defined in every study. There must be an independent variable and an dependent variable, the horizontal axis belongs to the independent and the vertical axis belongs to the dependent.
So, data is expressed as variables allowing to show it in the graph, each pair of variables represents a single data in the research. After graphing all points, we would a scatter plot expressing both axis an exact scale that allow to see if there is some relation between variables.
