There's a bridge but it was omitted for national security purposes.
(Isn't the boundary just what is set by the speed of light?)
Within that there's a smaller area defined by the limits of the human senses.
The line starting from the lower left ending in the upper right says: the earlier something happened the farther it cam be and remain physically observable (due to the speed of light being finite).
What humans can directly experience is of course small compared to the “all time” and “all space” and it would be invisible without the logaritmic scales used.
Felt it worth mentioning on the day Congress took the small (yet meaningful) step of legalizing industrial hemp production in the US.
Minute Physics SpaceTime Globe (The best tool for understanding relativity I've ever seen.)