Well, on the surface, it seems like OP's question is a good one. We would ALL want to know what the RDSON is for, say, any FET that is driven to supply rails, at certain levels of supply voltage, let's say.
But in practice, Vds is usually not going to be equal Vsupply, except when the transistor is fully off. Vds will be the voltage drop across the FET in series with a load - and therefore, Vds is going to be much less than Vsupply when switched on in any typical FET application (usually on the order of 1% of supply voltage). Perhaps this is why RDSon is typically shown in a datsheet as a function of Id rather than Vds. Vds could be used, because current and voltage are two sides of the same coin, but it would be potentially easy to confuse Vds with current supply voltage (or with max Vds) in this context, and it's simply a confuscation from the more common purpose of wanting to know this information. (In fact, this is the mistake I made in my first response to this thread).
Now that I think I'm reading this question correctly, I'm left wondering if this is not the intended question, at all. Vgs = Vds will pretty much happen only in one point, somewhere mid-switching, and it is sort of an arbitrary point where I fail to see the purpose of the curiosity. RDSon will be high at that point, and the situation will be unsustainable, anyway. The exact voltage where they will fleetingly match up will depend on the characteristics of the transistor, the supply voltage, and the load. And there will be only one point per set of variables where they cross... so the chart that the OP wrote doesn't make any sense, because who's to say that at Vgs = X you can even get Vds to also equal X (without changing load and/or supply voltage). Vds will be dependent on Vgs, Vsupply, load, and as someone else ponted out, temperature.
Or I'm totally in the dark. This question is very confusing and potentially misleading.