Graphic novels, as a niche market, have stabilized the past couple of years.  It takes something really different to catch the eye of a reader these days.  The American Library Association website should have the demographics you are looking for.  I teach (whenever my University forgets I don't own a PhD) a course on the Literature of the Graphic Word in which we study comics, comix, graphic novels, and such, and as the usefulness of graphic literature in the classroom becomes better accepted, I suspect the market will increase.

Personally, I don't buy digital comics.  You never actually own them and there are too many ways to electronically set them to self destruct, as some publishers are now wanting to do.  Go over to Kickstarter and check out all the graphic novel projects there. These are people much like yourself wanting to get started in the business, and the ones I have met in person are very generous with their information.

Let me know when you get something together.  I'd like to take a look at it.