I'm not sure that's reasonable - The effort parameters are integers between 1 and 10, with behavior described here: https://github.com/libjxl/libjxl/blob/main/doc/encode_effort..., the intermediate options don't exist as implemented programs. This is a comparison of concrete programs, not an attempt to analyze the best theoretically achievable.
Also, the frontier isn't convex, so it's unlikely that if intermediate options could be added then they would all be at least as good as the lines shown; and the use of log(speed) for the y-axis affects what a straight line on the graph means. It's fine for giving a good view of the dataset, but if you're going to make a guess about intermediate possibilities, 'speed' or 'time' should also be considered.
You are right, but that would make an uglier plot :)
Some of the intermediate options are available though, through various more fine-grained encoder settings than what is exposed via the overall effort setting. Of course they will not fall exactly on the line that was drawn, but as a first approximation, the line is probably closer to the truth than the staircase, which would be an underestimate of what can be done.
Also, the frontier isn't convex, so it's unlikely that if intermediate options could be added then they would all be at least as good as the lines shown; and the use of log(speed) for the y-axis affects what a straight line on the graph means. It's fine for giving a good view of the dataset, but if you're going to make a guess about intermediate possibilities, 'speed' or 'time' should also be considered.