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.Garbs....................................https://www.cgarbs.de _../|_ ='__ _~-. \' ~-`\._ |/~` _______________________________________________ rrd-users mailing list rrd-users@lists.oetiker.ch https://lists.oetiker.ch/cgi-bin/listinfo/rrd-users