For a line graph, it isn't possible to have a value
associated with y-axis itself. That is, if you have a
series of values along the x-axis, you cannot have a
value associated with x=0.
This makes the graphs look a bit strange when the
value at the first x tick (call it x=1) has a high y value.
Perhaps it is just personal preference, but it did appear
odd looking to me. I've attached a file that shows a
screen shot which might further describe it.