There are two ways to compute r 2 when the regression line is constrained. With constrained regression, there are two possible null hypotheses. One is a horizontal line through the mean of all Y values. The other null hypothesis would be a horizontal line through the origin, far from most of the data. Because r 2 is ambiguous in constrained linear regression, Prism doesn't report it.
Prism reports the P value testing the null hypothesis that the overall slope is zero.
The P value is calculated from an F test, and Prism also reports the value of F and its degrees of freedom. Two curves surrounding the best-fit line define the confidence interval. The dashed lines that demarcate the confidence interval are curved. This does not mean that the confidence interval includes the possibility of curves as well as straight lines. Rather, the curved lines are the boundaries of all possible straight lines.