
0 is really a logarithm and it's arithmetic value is "1". It's not arithmetic "No Exposure". You can have 1 exposure on that scale, which would be a tenth of the reference.
If the absolute measure were meter candle seconds, then where you sometimes arbitrarily assign 0 might be close to 3 log mcs. Again it's arbitrary. In your example 0 moves around. That helps to keep your graph simple.
Stephen, I assume you place 0.00 on the right and 2.75 on the left of the xaxis, and it is a log measure of the "attenuation" from full exposure. 0 log mcs is 1 arithmetic mcs. 1 arithmetic mcs is a lot of light. A film which requires that much light to develop to 0.1 net density  if my graph is correct  is EI 0.7. 3.0 log mcs corresponds to the 0.1 speed point of EI 800 (Have I got my graph aligned correctly?).
The arbitrary relative scale that reads 0 at the left and ascends 1, 2, 3 to the right, is setup that way just to be easy to visualize. A typical log mcs scale would read something like 3, 2, 1, 0 from left to right. But the arbitrary scale doesn't necessarily place its 0 where the log mcs scale has 3 it's just close to it sometimes.