I'll start by saying I don't know of any off-the-shelf graphing solutions
for visually impaired users. So everything that follows is sort of "what if"
hypothetical stuff, just thinking out loud.

Presenting data to blind and visually impaired users seems like a mostly
uninvestigated area, wide open for new research. There is an annual academic
conference called IEEE Vis: http://ieeevis.org/. Historically, their focus
is to present new research in different ways to visualize data. As far as I
know, they have never bitten off the challenge of presenting data in
non-visual ways. But if you have any pull in the academic community, perhaps
you could get some grad students to research ways to display data
non-visually, with the end-goal of getting their research published through
this conference.

Just thinking off the top of my head, I could imagine a bar chart with items
left-to-right (along the x axis) and values along the y axis. We could use
JAWS to read the value for each item, but instead of just reading numbers,
modulate the JAWS speech pitch in accordance with the value: larger values
would be read with a higher pitch, smaller values with a lower pitch. So,
instead of just hearing a bunch of numbers, you'd hear pitch changes with
each number that would allow the user to immediately place the value
relative to other values. The pitch changes would give you a "picture" of
the values relative to each other.

I'm sure that's a pretty naive approach, but as far as I know, no one has
tried it yet. It might be an interesting challenge for the JAWS scripting
language.
-Paul