How to prepare a presentation if there is a visually impaired person in the audience?
From conversations with my wife, who is blind, you don't want to overthink this too much. Most of the things you can do to help the visually impaired will help everyone in the audience.
For starters, I'd suggest sending out the slide deck beforehand if at all possible, since this helps everyone be prepared. You should also make sure any figures have some sort of caption or title on them with a brief description of what's being shown. This will help anyone using a screen-reader to follow along.
The major thing that will help is making sure that you're giving general descriptions of the charts and figures, as well as the takeaway from those figures. This is good practice in general, since it ensures that your audience understands what you're trying to show with a figure and is invaluable to someone with low vision because it allows them to better understand the information being presented.
The true goal is to ensure that someone with only an audio recording of your presentation, no slides at all, can follow along to a reasonable extent. A couple of things may be missed, but the overall meaning should be apparent.
From experience with someone who is visually impaired (but not blind): don't worry and go ahead as usual. Ask the visually impaired person what works best for them, if that is feasible. The person might want to use his/her own computer to look at your presentation, sit aside with at a special screen, or whatever works for them. One thing that would help in many cases is to use the mouse pointer instead of a laser pointer, since the former would be visible on the extra screen.
Figures and drawings with little text is best practice for presentations generally. Audiences can't really listen AND read full sentences at the same time, so you only want to use text in the form of signs and labels. The rest of the content should be either in your talk or in a handout available after the presentation (so they're not distracted by it during the presentation).
As for accommodating impairment, for colorblindness specifically check out this article: http://stephanieevergreen.com/handling-colorblindness/ (Stephanie Evergreen is an expert on presentations in general and I highly recommend reading more of her material)
Most other impairments are either near impossible to account for, or the person with the impairment will accommodate themselves because they have to for everything else anyway.
One caveat is when you have a regular attendee with an impairment that's specifically asked for help being accommodated. In such a case try to get a good understanding of what they specifically can and can't see and do your best from there. I don't think I'd go to the extent of running all my slides by them, but if you understand the disability well enough you shouldn't need to anyway.