If I have to graph 2 items, who do I know which is x and...

[kassimer]

If I have to graph 2 items, the hours students study and their grades, how do I know which item is x and which is y or does it matter?

[stapel_eliz]

If I have to graph 2 items, the hours students study and their grades, how do I know which item is x and which is y or does it matter?

I don't suppose it would matter, unless you're positing some sort of relationship, such as "grades increase when students study more". In that case, you'd want the input to be on the x-axis and the posited output to be on the y-axis.