I've pretty much ruled out the following things:

- my predictors are centered correctly
- I have calculated the regression lines both by hand and by computer, and get the same negative values
- I (and my advisor) have checked my regression output to the values I used to plot, and I didn't use the wrong values
- Missing data values aren't messing up my variables

I have figured out that the problem is probably with the negative constant in the regression equation. I don't think I should have a negative constant after I've centered my predictors, but I do. Oh, and I didn't accidently center the teacher literacy outcome, so it's not that.

Any ideas?