Pivot tables: can't add calculated item on grouped fields

Hello,
I saw that this problem is known but could not find a workaround or a solution.

I wanted to make a simple pivot table to track incomes/expense etc.
I made a sheet where I add my transactions divided by categories (for example salary) day by day.

Creating pivot table I used date as columns and categories as rows.
Automatically it groups date by month and years.

To add the calculated item (example net = income-expenses) I had to ungroup the dates.

After that I would like to group again by month and years but it's not possible anymore after having added the net item.
Is there a way to obtain this result?

Here you can find an example.

Thank you for this solution, it is exactly what I was looking for.

Is there a way to add other formulas like the net you added? For example I can have a formula to sum up some kind of data and a second formula to consider others.
Like to have multiple of the net that you add.

Thanks, I was able to add the second measure as you suggested, but when I add the second measure it is shown only in the columns.
For example using Net and net2 I have both on colum and only Net in rows.
Is it possible to set both of them on rows?

To make it clearer I would use this example. I want to add different formulas at the bottom row such as

1 Investment Gain1 = Value at Qtr2- value at Qtr1 and so on (could be negative->loss)
2 Investment Gain2 = $ Deposit - $ Whitdrawal (could be negative ->loss)
3 Total Income = Salary + IG1 + IG2
4 Tax = 0.3*Salary + 0.26*IG1
5 Net Incomes = Total Income - Tax

Expenses is a category so no need for formulas

6 Net Total = Net Income - Expenses

Then I would like to show all these intermidiates formulas from 1 to 6 and plot in a bar chart:
Bar 1 positive = Net Incomes
Bar 2 negative = Expenses
Then a line that represent the Net total

The problem is that your solution seems working fine untile I only had one formula. After adding different formulas it add the other formulas as field (column) and only the first one is computed in the rows.
That's why I was looking for a way to add calculated item because with calculated item I'm able to do that but then I have to forget about grouped field (data as Year/Qtr).

Is there a solution to do this?
Can I use more than one fomrula as you said but in the rows?

Thanks for your patience.