The curve has to be flat at zero speed. The curve can be extended into negative airspeeds, and for low negative airspeed it should be a mirror image of the low positive airspeeds. Continuity of the curve requires it to be flat at zero speed.
What happens between zero speed and translational lift speed is a decrease in induced drag, since the forward speed increases efficiency of the disk. How much the efficiency increases will spell the difference between a fairly constant line and a sharply decreasing line.
It's important to remember that the points on the curve are for steady state. I confused myself once thinking you created more induced drag as airspeed increased from zero because you still need power to maintain level, but you also need power to accelerate. That is not a steady state thus I confused myself.
SUM UP!! Level at zero, decreases towards TL, gradually decreases until you get into higher speed compressibility issues.