The chart was prepared by Aurora Flight Sciences, an aircraft manufacturer, commissioned by a professor who supports the concept of maintaining a fleet to pump sulphuric acid into the stratosphere as a way to induce artificial cooling to counteract human-induced global warming.
The chart appears to compare many different ways of shooting the acid into the skies along two dimensions: cost and altitude.
I find the choice of axes extremely counterintuitive. Altitude one would expect on the y-axis. And mixing up the scatter chart elements with the connected line chart doesn' really help either.
The convention regarding axes is to put the outcome variable on the vertical, and the explanatory variable on the horizontal. Thus, in this case, if the cost of a particular solution is primarily determined by the "altitude" (presumably of where the acid would be released), then the designer has followed convention. It is unfortunate that "altitude" is more intuitively put on the vertical axis, but I suspect that defying convention might cause more confusion.
On the other hand, if altitude and cost are not related to each other but two different metrics to evaluate geoengineering concepts, then Bernie's point is right on - swap the axes!
The use of connected lines for two of the solutions but not the rest is a symptom of what I have called "loss aversion". The horror of leaving some of the data on the cutting floor.
The only mention of altitude in the article refers to Aurora's assertion that it is sufficient to use newly designed aircraft flying at 20-25 kilometers. If that is Aurora's preferred solution, there is little reason to show all the other altitude configurations that are suboptimal.
Perhaps the designer wants to make the point that the Boeing 747 solution is inferior to the Aurora solution because Aurora could design aircraft to fly at 10-15 km at a lower cost? If so, then the chart is very misleading in not providing a comparable cost for Boeing's solution if required to fly at 20-25 km.
When comparing different entities, it is always a bad idea to treat the entities differently. Comparison is only possible on equal footing.
In fact, I think the chart would be a lot clearer if they dropped the altitude dimension on the floor. For each solution, plot the yearly cost at the optimal altitude selected by the respective engineers. Use a bar chart. With a single dimension, it is much easier to accommodate the very long data labels.
(Now, I'd defer to the geoengineers as to whether the altitude dimension is dispensable. I don't have any expertise in this science. Judging from the Aurora red line, I'm assuming that there can be feasible solutions at all altitudes, which leads me to conclude that altitude isn't all that.)
So what is the biggest problem with this chart? It is the faux accuracy.
Given the tremendous amount of uncertainty surrounding these projected costs, one would expect very big error bars around the cost estimates. Using single dots with no error bars is hard to stomach.