On the day a new position is entered into a portfolio, the Chg$ column uses the previous day's closing price to calculate what it displays and gives an inaccurate reading. This value should be based on the entry price when the purchase date matches todays day.
For example, I initiated a short position today of 9K shares at $7.70. It closed yesterday at $7.28. Currently the Last column says $7.46, the Gain $ column shows $2,180 and the Chg$ column shows -$1620 because it's calculating from 7.28 instead of 7.70.