The reasoning is simple. A warming is said to be due to the "greenhouse effect", the major portion of which is due to "water vapor in atmosphere". If you show me a chart of "greenhouse gas concentration" and leave out water vapor, something is not making sense.
I don't know whats going on with that chart, whatever. I am just telling you the end result of showing it to me, and doubt I am alone.
The meaning and purpose of the chart is extremely clear. They're varying C02 and similar gasses, and looking at what happens. Water vapor is held constant as a first order effect and so isn't plotted.
I don't draw straight lines on all my plots relating constants to (in)dependent variables. Even fifth grade science experiments follow this convention in their plots. It's an extremely common practice. Water vapor isn't being varied, so isn't plotted. Again, nearly every fifth grade science fair project follows this convention in their plots. It really does count as basic literacy.
For someone with some basic scientific literacy, the author's intent is obvious and confirmable by reading the literature. OBVIOUSLY the authors are aware of water vapor concentrations, and you can confirm this awareness with a superficial investigation into how this metric is used in models.
Sorry, but I do not believe a non-antagonistic reader would jump to the conclusion that climate scientists are unaware of tyoical atmospheric H20 concentrations. That's so far from the realm of reasonability.
At some point, the readers' good faith must be assumed or the entire expository enterprise is pointless. I think this plot is by far on the correct side of that line.
Yeah, warming increases atmospheric water vapour slightly so a simplified model that assumes it remains constant is going to slightly underestimate the total warming caused by a given amount of non-water-vapour emissions. Models don't have to take into account every detail to be useful, though.
I don't know whats going on with that chart, whatever. I am just telling you the end result of showing it to me, and doubt I am alone.