Correlation of USPTO Art Units with IPC subclasses

What correlation, if any, is there between the art units to which the USPTO allocates patent applications for examination and the International Patent Classification subclasses allocated to the claimed inventions encompassed by the resultant patents? One would expect a fairly strong correlation, given the USPTO’s allocation of US patent classifications to art units and further in view of similarities between the IPC and US patent classification schemes.  For background, see the USPTO’s tabulation of US Patent Classes Arranged by Art Unit and its US patent classification to IPC concordance.

This scatter plot shows, along the horizontal axis, numbers of US patents which issued in 2012 for different USPTO art units; and along the horizontal axis, numbers of those patents in terms of their primary IPC subclasses; both of those measures being referenced to the assignee dimension (i.e. the parties to whom the patents issued).

The positive slope trend line reveals a strong positive correlation, as expected. The underlying trend model has an r² value of 0.843717, so r (the correlation value, in accordance with the Pearson correlation) is 0.92. Correlation values close to either +1 or -1 represent strong positive or negative correlation respectively between the measures being compared. Correlation values closer to zero represent weaker—or an absence of—correlation.