Determining which data goes along the x-axis and which data goes along the y-axis.
You are told to measure the height of the water in an aquarium every 20 seconds while your partner is pouring water into the aquarium. Your graph will have the height of the water plotted against the time...but which set of data goes along the x-axis and which goes along the y-axis?
The x-axis is your "known" before the experiment. Before you started the sample experiment, you knew you would be taking a measurement every 20 seconds; this is your know. So time will be placed along the x-axis.
The y-axis is your "unknown." Before you started the experiment, you had no idea how high the water would be after 20 seconds, 40 seconds... So, in this case, height of the water is placed along the y-axis.