add support for negative logarithms

[mmitch]

Define a signed logarithm function that works like this:

   x >= 0 : slog10(x) =   log10(  x )
   x  < 0 : slog10(x) = - log10( -x )

This will allow drawing both above und below the X axis when using logarithmic scale.

Current limitations:

• Y autoscale does not detect negative values, --lower-limit has to be used
• Y axis labeling is missing

Can anybody help me to solve these problems?

[mmitch]

see this script for a working demonstration: https://gist.github.com/mmitch/6a7e4bda057d0b4ae2d46515f5a30f7a

[dword1511]

Doesn't slog10(0.1) and slog(-10) get the same value?

[mmitch]

Strictly mathematically speaking: yes. But I have not yet seen an rrdgraph logarithmic diagram with negative values. I think the values are autoscaled in such a way that x never is less than 1 (at least the numeric value of x - the axis legend will happily show a scale of 1/10 or 1/1000 using prefixes like m or u).

[oetiker]

if memory serves I am cutting values <=1 since there is a discontinuity