A long forward payoff is a straight line in the terminal spot price.
highlighted = computed this step
Locked delivery price
A long forward locks in delivery at $105.00 and pays terminal spot minus the forward price at maturity. It has zero cost to enter in this model.
long payoff=ST−F0
Straight-line payoff
At terminal spot $90.00 the payoff is a loss of $15.00; at $105.00 it is $0.00; at $120.00 it is a gain of $15.00. The line crosses zero at the forward price.
ST−F0∈{−$15.00,$0.00,$15.00}
Put-call parity tie-back
The diagram reuses the payoff primitive from Option Payoffs. A long call and short put at the same strike make the same straight-line payoff by put-call parity.