Re: [rrd-users] rrdgraph: Negative values with --logarithmic
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.Garbshttps://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) ___ rrd-users mailing list rrd-users@lists.oetiker.ch https://lists.oetiker.ch/cgi-bin/listinfo/rrd-users
Re: [rrd-users] rrdgraph: Negative values with --logarithmic
On Mon, Oct 17, 2016 at 03:01:11AM +0200, Alex van den Bogaerdt 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.
>
> So, what you want to show is the outcome of a normal logaritmic function,
> but mirrored. Instead of a new function...
Basically, yes, but because of the y axis I think I need some special
logic in rrdgraph (see below).
> > 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.
> >
> > Think of something like a "display-logarithm" function defined like
> > this:
> >
> >
> > double disp_log(double y)
> > {
> > if (y == 0) {
> > return 0;
> > } else if (y > 0) {
> > return log(y);
> > } else {
> > return -log(-y);
> > }
> > }
> >
> >
> > While "-log(-y)" might look weird in a mathematical context I think that
> > it makes sense in describing "do the same as for positive values, only
> > in the other direction".
>
> ... can't you just use CDEF:mirrored=logresult,-1,* ?
> Or am I missing something ?
This would indeed give me the correct results and would draw a correct
graph, but the y axis grid would not be logarithmic:
If I put the values "1000" and "-1000" into a CDEF using normal and
'mirrored' LOG, I'll graph the values "3" and "-3". The y grid will
propably autoscale to "-10 … 10" and show my values as "3" and "-3"
instead of "1000" and "-1000".
When I use --logarithmic, the positive values are also LOGed, but the y
axis will be scaled logarithmic as well, so the y axis will show
"1000" (or "10e3" or "1k") next to the graph value.
If I could get a logarithmic y axis without using --logarithmic (for
both positive and negative values), using LOG in a CDEF would indeed
be a solution.
Regards
Christian
--
Christian.Garbshttps://www.cgarbs.de
"Die Berühmtheit mancher Zeitgenossen hängt mit der Blödheit
der Bewunderer zusammen."-- Heiner Geissler
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Re: [rrd-users] rrdgraph: Negative values with --logarithmic
> 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.
So, what you want to show is the outcome of a normal logaritmic function,
but mirrored. Instead of a new function...
> 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.
>
> Think of something like a "display-logarithm" function defined like
> this:
>
>
> double disp_log(double y)
> {
> if (y == 0) {
> return 0;
> } else if (y > 0) {
> return log(y);
> } else {
> return -log(-y);
> }
> }
>
>
> While "-log(-y)" might look weird in a mathematical context I think that
> it makes sense in describing "do the same as for positive values, only
> in the other direction".
... can't you just use CDEF:mirrored=logresult,-1,* ?
Or am I missing something ?
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[rrd-users] rrdgraph: Negative values with --logarithmic
Hi there,
looking through the mailing list archive, I found several requests
like mine over the last years:
"Why won't negative values show up in my graph when I use --lograrithmic?"
The answer was mostly that a logarithm is not defined for negative
numbers.
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.
Think of something like a "display-logarithm" function defined like
this:
double disp_log(double y)
{
if (y == 0) {
return 0;
} else if (y > 0) {
return log(y);
} else {
return -log(-y);
}
}
While "-log(-y)" might look weird in a mathematical context I think that
it makes sense in describing "do the same as for positive values, only
in the other direction".
I have had a look at rrdgraph.c and think that I found the calculation
of the final y value for a given data point in the ytr() method. But
I did not yet fully figure out how im->minval is computed (im->minval
seems to be the reason for negative values to be cut off).
(And if this makes sense at all, it should be probably be controlled
by a commandline parametera and I have not yet started to look what
else needs to be changed then: commandline parsing, internal
configuration state, manpages, documentation...)
What do you think of this?
Would the be a useful feature?
Could this somehow be implemented?
Regards
Christian
--
Christian.Garbshttps://www.cgarbs.de
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