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Very interesting. I reckon for completeness it'd be handy to also post the histograms at different exposures, so you can relate the different parts of the above charts to histograms shapes/peaks.
I don't know much about CCD sensors, but if this were a DSLR's results I'd be expecting something similar in terms of the shapes of those charts. The min values just increasing linearly but the max values dropping off. Think it's to do with sensors being more sensitive at the highlight end. Similar to the "exposing to the right" philosophy in photography.
I've read about the recommendations for flats being to have the peak of histogram around 1/3 to 1/2 way from the left (darks) end. Be interesting to do the histogram comparison as I suggested above and it may just be that this relates to that linear part of the curves up to 7 or 8 secs anyway. Hence both theories/recommendations are correct.
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