[rrd-users] rrdgraph: Negative values with --logarithmic

[Christian Garbs]

On Sun, Oct 16, 2016 at 09:31:18PM +0200, Christian Garbs wrote:

> What I want to achieve (and what I think those other proponents also
> had in mind) is that I essentially have some of my data mirrored at
> the x-axis for better readability (don't draw two values on top of
> each other).
> 
> For example I plot outgoing packets on my network as a positive values
> while I plot the incoming packets via CDEF:output=0,outputx,- as
> negative values.  So the outgoing rate is shown above the x-axis and
> the incoming rate below the x-axis.
> 
> To tame network spikes somewhat I want to change the graph mode to
> logarithmic.  This works for the positive values, but the negative
> values simply vanish.  I just want the same transormation that happens
> to the positive values applied 'mirrored' the the negative values.

I now have a first working prototype, see
https://github.com/oetiker/rrdtool-1.x/pull/757

I can plot a "negative logarithm" just like I described.

There are still two problems:

 - Y axis legend is not drawn
 - Y axis autoscale does not work, you have to explicitely set --lower-limit

Would anybody with more knowledge about rrdgraph.c like to have a look
at these problems?

Thanks!
Christian
-- 
....Christian.Garbs....................................https://www.cgarbs.de
"I can't believe you don't believe me."
(Note: this tactic is successful in focusing the other person's
 attention away from the subject)