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.