When the reality has to be shown in a diagram, it's very seldon straightforward. One should be quite happy if any kind of functional connection can be recognised. At best, this would be a little over 50% in the first part of the curve.
It is also important that both axis be properly chosen. In this case therefore, the mixing proportion is shown in percent on the X-axis and the resulting antifreeze temperature on the Y-axis. Since the mixing proportion is given or independently changed and the respective antifreeze protection depends on this. It would be completely senseless to do it the other way around.
The actual message in the diagram, is that one can do too much of a good thing. The antifreeze temperature sinks, in an almost linear fashion, up to just under 60 %, and rises again, when antifreeze is added, up to -15°C. Indeed, those who travel with pure antifreeze in the cooling system, have a protection which is insufficient, even for central European conditions.
Water, by the way, has an especially good ability to transfer heat, this is however, very much reduced when using large amounts of antifreeze, this can cause, not only the freezing of the cooling system on cold nights but when straining the engine while driving, it can also result in overheating. Here it's a case of spending more to get two disadvantages at the time.
Don't say, this can't happen to you. Okay, probably not, if you're always on the road with relatively cars. Indeed, what about those cars that continuously lose a little coolant? If you always top up with only water, the antifreeze protection suffers, and if you take special care by topping up only with antifreeze, the cooling system is going to suffer, sooner or later, as well. 06/11