What you are describing is more a specialised drawing tool. Graphs are
about converting numbers into pictures.

If this were my problem I would look into how you detect mouse move,
drag at a low level, and just draw the bars with fillRect on a JPanel
in paintComponent.

See http://mindprod.com/jgloss/event11.html

If you can't find something, and if working at that level leaves you
cold, I would be happy to create such a tool for you for a flat fee.

Let's say you were using this tool to track your daily weight, or hit
counts for example, the problem with drag is it is crude. It could be
very frustrating to the user to not be able to get it bang on the
value he wants.  You might want to have some fine-tuning mechanism to
inch the bar up or down, with a numeric indicator.

You also might want some auto-scaling logic so as a bar grows off the
top the whole graph scales to make it possible to display the desired
value.

If you have legends you want scaling to happen in various pretty
units.