snip

When I look at the graph, I see classic straight line data.  No biological
measurement has exact measures, but it is obvious from the graph that the MZ
twins have much more similar refractive error than the DZ twins.

Thanks for the link to the actual study, here is the conclusion; about 90%
of the refractive error can be explained by inherited factors, the rest
attributed to environment:

"RESULTS. For the continuous spectrum of myopia/hyperopia, a model
specifying additive genetic and unique environmental factors showed the best
fit to the data, yielding a heritability of 84% to 86% (95% confidence
interval [CI], 81%-89%). If myopia and hyperopia ( -0.5 D and  0.5 D,
respectively) were treated as binary traits, the heritability was 90% (95%
CI, 81%-95%) for myopia and 89% (95% CI, 81%-94%) for hyperopia. For total
and corneal astigmatism, modeling showed dominant genetic effects are
important; dominant genetic effects accounted for 47% to 49% of the variance
of total astigmatism (95% CI, 37%-55%) and 42% to 61% of corneal astigmatism
variance (95% CI, 8%-71%), with additive genetic factors accounting for 1%
to 4% and 4% to 18%, respectively (95% CIs, 0%-13% and 0%-60%,
respectively).

CONCLUSIONS. Genetic effects are of major importance in myopia/hyperopia;
astigmatism appears to be dominantly inherited."