"lines" attribute format?

[jfirebaugh]

The README suggests that in the report format, the value for :lines is a hash of line number to sample count (e.g. :lines=>{12=>91}).

But in the reports that I've generated, it seems to be a hash of line number to two-element arrays (e.g. :lines=>{12=>[91,0]}). What do the two array elements mean?