[mrtg] What is the scale of for logscale?

[Fred Zwarts]

Recently we upgraded to MRTG 2.12.2, which has the logscale option.
I tried it and indeed the vertical scale changes.
But I wonder what scale is actually used.
I was surprised that there is always a 0 at lower end of the Y axis,
although a 0 cannot be shown on a true log scale.
E.g, I have a graph with on the vertical axis the following numbers
at the tick marks (from top to bottom): 
5.1 M, 1.6 M, 0.5 M, 0.1 M and 0.0 M.
I would expect a constant factor between these numbers for a log scale,
which seems to be something close to 3. However, in that case the
number at the low end should have a value of about 30 k.
This would explain why on many graphs I still do not see the background,
but only the peaks. It seems that everything 200 times smaller the
highest peak is not visible.
I would like to have a greater factor between the number at the tick marks,
so that the background becomes visible.

My questions are:

Is a real log-scale used, or is it an approximation with a few linear 
segments to save computing power?
How is the factor between the tick marks calculated and can this be changed?