Consider the graph shown.

a graph is shown in the xy-plane. the values on the x-axis ranges from 0 to 1 decimal point 5 in increments of 0 decimal point 5 and the values on the y-axis ranges from 0 to 800 in increments of 200. a line starts from a point 100 on the y-axis, goes up, and passes through the points (0, 100), (0 decimal point 3, 200), (0 decimal point 9, 400), and (1 decimal point 5, 600).

determine if the graph shows two quantities that vary directly. if possible, determine the constant of proportionality. explain your reasoning.