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 x-axis, 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.