register --average is non-optimal with multiple accounts per period

[simonmichael]

t.j:

1/1
  (a)  10
  (b)  20

2/1
  (a)  30

hledger's (and ledger's) register --average shows the running total divided by the number of lines displayed:

$ hledger -f t.j register --monthly --average
2014/01/01 - 2014/01/31         a           10            10  # total / 1 row
                                b           20            15  # total / 2 rows
2014/02/01 - 2014/02/28         a           30            20  # total / 3 rows

Wouldn't it be more useful to show the running total divided by the number of periods, eg:

$ hledger -f t.j register --monthly --average
2014/01/01 - 2014/01/31         a           10            10   # total / 1 month
                                b           20            30   # total / 1 month
2014/02/01 - 2014/02/28         a           30            30   # total / 2 months