The Orange data frame contains 35 rows and 3 columns of records of the growth of orange trees.

The Three columns are:

• Tree: an ordered factor indicating the tree on which the measurement is made.The ordering is according to increasing maximum diameter.
• age: a numeric vector giving the age of the tree (days since 1968/12/31).
• circumference: a numeric vector of trunk circumferences (mm) at “breast height”.

Below is a scatter plot of the age versus circumference, and the fitted line is from the linear model.

It is clearly that the age and the circumference are positively associated.
One thing to note, the variance of the circumference also increases when the age increases.