Speed is the magnitude that relates space and duration indicating the distance traveled in the time unit.
Do the trajectories exist in the space-time graphics really?
"You're still at 3 feet from me. Your "trajectory" in the space-time graphic is a straight vertical line shown in the figure".
X axis: distance (meters). Y axis: time (seconds).
The trajectories in space-time graphics are not real, they are only observable if the observer is outside of time, and no observer can be outside of time. In the graphic, the time of the observer and the subject pass equal, parallel. This also applies to the curvature of space-time graphic. The curvature of light when passes near the sun is a real phenomenon and different to the curvature of space-time graphic. The light is bent by the sun gravity. This is a spatial curvature, and not of the space-time.
MODIFIED GRAPHIC
"You're still at 3 feet from me (green). Your "trajectory" (red) in the space-time graphic is a straight vertical line shown in the figure".