```r
boxplot(fitted~predictor,data=d)
abline(h=mean(results),lwd=3,col=2)
arrows(1,10.66,1,mean(results),code=3)
arrows(2,13.24,2,mean(results),code=3)
arrows(3,19.33,3,mean(results),code=3)
```

<img src="Quantitative_and_Spatial_files/figure-html/unnamed-chunk-131-1.png" width="672" />