Add monthly cash flow example report

docs/lua_reports.md

@@ -42,6 +42,8 @@ function generate()
 end
 ```
 
+More examples can be found in the reports/ directory in the MoneyGo source tree.
+
 ## Basic Operation
 
 The lua code behind a report *must* contain a `generate()` function which takes

reports/monthly_cash_flow.lua

@@ -0,0 +1,26 @@
+function generate()
+	year = date.now().year
+
+	accounts = get_accounts()
+	t = tabulation.new(12)
+	t:title(year .. " Monthly Cash Flow")
+	series = t:series("Income minus expenses")
+
+	for month=1,12 do
+		begin_date = date.new(year, month, 1)
+		end_date = date.new(year, month+1, 1)
+
+		t:label(month, tostring(begin_date))
+		cash_flow = 0
+
+		for id, acct in pairs(accounts) do
+			if acct.type == account.Expense or acct.type == account.Income then
+				balance = acct:balance(begin_date, end_date)
+				cash_flow = cash_flow - balance.amount
+			end
+		end
+		series:value(month, cash_flow)
+	end
+
+	return t
+end