While the author included income from dividends in his extensive examples, i can't find "investments" under the expenses categories.
Oversight? Hard to imagine, given the presence of some form of investment return on the income side. And clearly the money invested is not money going to the bank, waiting to be put to work.
If i do mark it down as expense, though, the gulf between income and expenses will look far worse than it would otherwise, which might demotivate me.
I thought of including a second, right-hand y-axis for total assets to alleviate the problem.
It really depends on what you're measuring.
For example, technically, the principal portion of a mortgage payment is a transfer of one asset (cash) to another asset (property equity). So, from a net worth perspective it's not an expense, it's a transfer, like swapping stock for bonds and vice-versa.
From a monthly, "I've got to pay my bills!" cash flow perspective, it sure as hell IS an expense. It's got to be paid.
So, what are you trying to measure?
For example:
How much are my monthly expenses changing?
How much are my savings changing?
How is my net worth changing?
How is my passive income changing?
How is my passive income compared to my planned FIRE expenses?
Different things will be included to properly answer those questions.
Decide on the question first.