When you speak alot you run into some crazy room setups. It's your job to be there early to make sure all the little preparation details are taken care of. One of these details is the visibility of you and your visuals from each chair in the audience.
It doesn't even amaze me any more that a hotel room setup crew would put up a projection screen that 1/3 of the audience couldn't possibly see. Most of the time this occurs when tables and or chairs are placed too far forward in the room. The people on the sides are at much too sharp an angle to see anything on the screen and they only see you in profile.
Another situation is when there are pillars in the room. The setup people normally just slap tables up to suit themselves and forget that the people sitting in the chairs just might like to see what is going on at the stage. When you have pillars to deal with you must be very aware of your stage positioning. You won't be able to be seen the entire time by everyone because of the pillars, but you can limit the time you are unseen by noting the worst areas of the stage with an "X" of tape on the floor. Avoid standing still anywhere near the "X." It's OK to passby or over the "X," just don't plant yourself near it.
It's your job to check every chair in the room to be sure every participant has an unobstructed line of sight for each part of your presentation. They must be able to see the screen, the flipchart and you wherever you travel on the stage.
If possible, be in the room early when lots of setup crew are around so that you can make the adjustments before the room is totally set. Taking care of details like this is one more vote for you as a highly polished presenter.