我想在下面的ggplot上显示R2值，如何实现？
ggplot(xray,aes(x=PercentGoodFish1g,y=PercentPopAff)) +
geom_point() +
labs(x='Good fish yield to 1g',y='Percentage fish with deformity',title='Good fish yield to 1g vs percentage fish with >1 vertebral deformity') +
geom_smooth(method ='lm')
