Not to disagree with you
Why not? This should not be a consideration.
this graph does not directly represent signal to noise ratio. It represents Dynamic Range
The "photographic" definition dynamic range (as opposed to the "scientific" one) is based on the acceptance of a certain level of SNR. This should not be the only criterion, but that's a different issue.
Anyway, the DR shown on this chart is based on the SNR limit 20, i.e. the standard deviation around the lowest accepted signal is 5% of the signal.
The chart shows for example re the D3, that the DR is reduced by one EV with every stop increase of ISO above 800. This means just, that the SNR doubles by increasing the ISO by one stop.
The consequence is, that ISO 200 with the D2X, ISO 400 for the D200, ISO 800 for the D300 in 12bit mode, ISO 1600 for the D3 and for the 1DsMkII are useless, except when creating JPEG images in camera. I can't test this for myself, but I find it hard to believe and it is certainly not so with my Canon 20D.
When the SNR increases only by sqrt 2 per EV, then the DR is reduced by 1 EV only per every two stop increase of the ISO. This is the result of my measurement.
If you have one of the cameras included in that chart, you can shoot test images and if you upload the raw files, I analyze them.