65 Years
65 Years
[A graph titled 'Number of Living Humans Who Have Walked on Another World' - its y-axis is numbered 5, 10, 15, its x-axis increments every ten years from 1960-2040. The line of the graph has a bracket above it that says '65 Years', starting at 1969, ending in 2034. The line starts at 1969 and increases steeply to 12 by 1972. It then plateaus until the early nineties, declines gradually to 9 between 1991-1999, and then plateaus again. From 2011-2035, which is labeled 'Projected Actuarial Tables', the line branches into three and begins to decline more steeply to zero. The area between the first and second branch is shaded and labeled '5th percentile' and the area between the second and third branch is shaded and labeled '95th percentile.']

Randall is showing the number of still living humans who have walked on another world for the 65-year period that begins in 1969 (when a human first walked on the moon). Up to 2011 (when the comic was drawn), he has drawn a single line for the actual figures.
For the subsequent years, he has drawn three lines using actuarial tables or life tables (such tables show, for each age, the probability that a certain person will die within the next year).
The line marked "5TH PERCENTILE" indicates that there is a 95% probability that the number alive in a given year will be above that line and a 5% probability that the number alive will be below that line.  For example, this line indicates a 5% chance that all Apollo moon walkers will be dead by 2023, and a 95% chance that at least one will still be alive by that year.
The line marked "95TH PERCENTILE" indicates that there is a 5% probability that the number alive in a given year will be above that line and a 95% probability that the number alive will be below that line.  For example, this line indicates a 95% chance that all Apollo moon walkers will be dead by 2035, and a 5% chance that at least one will still be alive by that year.
The middle line is not identified, but is probably the "50TH PERCENTILE" (see these tables ).  If so, it indicates that there is a 50% probability that the number alive in a given year will be above that line and a 50% probability that the number alive will be below that line.  For example, this line indicates a 50% chance that all Apollo moon walkers will be dead by 2028 (see previous link), and a 50% chance that at least one will still be alive by that year.
Although the term other world would include all other worlds on which humans have walked, there is currently only one other world on which humans have walked, which is the moon.  The humans that have walked there are the 12 Apollo astronauts who landed on the Moon between 1969 and 1972.
In particular, Neil Armstrong and Buzz Aldrin landed in July 1969. Pete Conrad and Alan Bean landed in November. Alan Shepard and Edgar Mitchell : February 1971. David Scott and James Irwin : July 1971. John Young and Charles Duke : April 1972. Eugene Cernan and Harrison Schmitt : December 1972.
Irwin died in 1991. Shepard and Conrad died in 1998 and 1999 respectively, making the total 9 as of the date this comic was published. Since then Armstrong died in 2012, Mitchell in 2016, Cernan in 2017, Young on January 6, 2018, and Bean on May 26, 2018. The current (April 2021) number is 4, which lies close to the middle line (the supposed 50TH PERCENTILE). The oldest living person to have landed on the moon is Aldrin at 91. Also living are Scott at 88, Schmitt at 85, and Duke at 85.
The chart assumes that no other humans will go to walk on another world within the time-frame plotted and the title text implies that this is primarily an economically determined decision. While noting that not exploring space is a justifiable and sensible decision which may also be made by many hypothetical cultures on other worlds, the text implies a grandness to a civilization that would be given the opportunity to discover, study and memorialize the 'one-world graves' of other civilizations by choosing to explore space despite the economic difficulty. This also implies that the likely consequence of not exploring space is that a civilisation which chooses to do this is doomed to go extinct fairly rapidly while those which do explore and colonise may last long enough to be safely established on multiple worlds and discover the remains of civilisations which acted on a purely economic basis and hence ensured their own collapse. High five for exoplanet archaeology.
[A graph titled 'Number of Living Humans Who Have Walked on Another World' - its y-axis is numbered 5, 10, 15, its x-axis increments every ten years from 1960-2040. The line of the graph has a bracket above it that says '65 Years', starting at 1969, ending in 2034. The line starts at 1969 and increases steeply to 12 by 1972. It then plateaus until the early nineties, declines gradually to 9 between 1991-1999, and then plateaus again. From 2011-2035, which is labeled 'Projected Actuarial Tables', the line branches into three and begins to decline more steeply to zero. The area between the first and second branch is shaded and labeled '5th percentile' and the area between the second and third branch is shaded and labeled '95th percentile.']
