Yours is an interesting situation.
There are couple of approaches based on the options you have.
Do you have represent every single record on the graph?
Let us say you have alloted 800 as the max you could get.
and you actually get only 600. One way of dealing with this is
split it into multiples that would also be multiples of the
max. So represent the first 400 as it is and for the next 200,
use a broader graph scale so you would drawing a line every alternate pixel.
If the actual exceeds, the max, you can represent every alternate intially with a narrow scale and then switch to regular scale and represent the most recent.
Here the assumption is that most recent is more important.
Well, my solution is a result of 2 mins of thinking so this is more of a quick response.
I hope this gives you some off the track info.