Steve,
The image circle is rated at 60mm. The chart shows from the centre of the lens out, so 30mm is the radius. The diagonal of a full frame sensor is 43mm, so half of that is 21.5mm. And 21.5mm is 71.7% of 30mm. So to figure out where the unshifted part of the 35mm sensor is, you move my dashed red line to the 71.7% position on the X axis of the chart. Everything from the dashed red line back to 0% is the image circle that covers the full frame sensor.
Next if you look at the labels you see that the lines represent 10 lp/mm, 20 lp/mm and 30 lp/mm; that's what the "spatial frequency" label refers to. Those are fairly conservative numbers. For example, if you dig out the corresponding technical document for the Apo-Digitar 80mm f/4, you'll notice they're using 20 lp/mm, 40 lp/mm and 60 lp/mm. The way I read that is Schneider-Kreuznach is telling us (obliquely) this is a lens from the film days and don't get your hopes up too high on a high resolution digital sensor! The curves are also telling me that corner performance might be OK stopped down if you don't shift, but shifting will make you unhappy. Expect soft, glowy corners.
I am already pushing the boundaries of my knowledge here (not being any kind of optics expert). Hopefully others will chime in who have a deeper knowledge. While you're waiting, search for a PDF of "How to Read MTF Curves" by Dr. Nasse from Zeiss. It should come up right away if you look for the document code Article-MTF-2008-EN. It's heavy going, but he does walk you through how to read these charts.
Finally, I did a quick search on Flickr for pictures made with the Leica 28/2.8 PC and some came up. If you're happy with what you're seeing from the Leica copy, you should be fine with the Schneider-Kreuznach because SK made the lens for Leica. It is the same lens as the SK version for the Leica R mount, and the SK version in Copal 0 -- so pictures from any one of those will do.
If you do get a copy of this lens and you're blown away by how great it is (meaning I'm dead wrong!), please let us know. I'd love to be wrong -- seriously. I have the Sammy 24/3.5 and it's actually really good at f/11, which is where you'd likely use a shift lens like that anyway. However, I rarely use it because 24mm is too wide for me. The nice thing about the Sammy is it's not hard to remount. I did it for my Toyo, and another person I know did it for his Cambo Actus. It mounted very easily to a Mamiya 645 plate, with next to no modifications needed; I was really impressed by how easy it was.