Answer:
the intercept is the orgin (0,0)
The question is unclear regarding the boat's velocity. Is it 20 m/s south relative to the water, or relative to the earth? (It is a river, after all...)
There's also the possibility that the boat's velocity relative to the river is 0. Take the south direction to be negative and north to be positive, and denote by
the velocity of a body A relative to a body B. Under these conditions,

(B for boat, E for earth, W for water) so that the passenger's velocity relative to the earth is

(P for passenger)

or 10 m/s south.
The velocity curve is such in which the x-axis is the velocity and the y is the time. The distance traveled by the object is calculated by multiplying velocity and time. Thus, in the velocity curve, the distance can be easily calculated if we know where to look.