The negative values on the graph actually represent the city's exports. When they are greater than 0, it indicates that the city is importing. When you build a port, airport or freight railway station, these freight stations will import certain goods to ensure that the warehouse has a certain amount of emergency inventory. That's why when you build them, your graph will rise and become positive. When these freight stations have imported their emergency inventory, they will stop importing goods, so your graph will drop to a negative value and return to the daily export value.