> str(stretch(d))
Classes 'tbl_df', 'tbl' and 'data.frame':	16 obs. of  3 variables:
 $ x: chr  "Sepal.Length" "Sepal.Length" "Sepal.Length" "Sepal.Length" ...
 $ y: chr  "Sepal.Length" "Sepal.Width" "Petal.Length" "Petal.Width" ...
 $ r: num  NA -0.112 0.871 0.817 -0.112 ...

