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.
This is another contradiction to what I've been told: "Satellites have observed an increase in atmospheric water vapour by about 0.41 kg/m² per decade since 1988." https://www.skepticalscience.com/water-vapor-greenhouse-gas-...
We aren't actively and directly melting large quantities of ice. It's a second-order effect of increased atmospheric C02.
One option is to allow it to vary but not as a first-order effect. Another option is to simply not let it vary to keep the model simple.
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.