Scientist tested the effect of a fertilizer on the growth of plants. they had 2 sets of plants, group a which gets fertilizer, water and sun and group b which gets only water and sun(but in equal amounts to group a).
the graph below was created showing the growth of the plant. how would you know that the y-axis is measuring and show the y-axis and x-axis

For every experiment conducted by scientists, there are usually independent variables and dependent variable. The independent variable is that parameter whose variation does not depend on that of another variable while the dependent variable is the one whose variation depends on the independent variable. 
In the question given above, the independent variable is the fertilizer used while the dependent variable is the growth of the plant.
Conventionally, independent variable are normally placed on the X axis of the graph while the dependent variable goes on the Y axis. Thus, the quantity of fertilizer used will be plotted on the X axis [Horizontal line] while the increase in the plant growth will be plotted on the Y axis [Vertical line].