Add margin to X-axis on RRDtool graph

Question

A customer of mine wants to get better insight of the dBm values that an optical SFP sends and receives. Every 5 minute I poll these values and update the values in an RRD file. The RRD graph I create with the RRD file as its source is created in the following way:

/usr/bin/rrdtool graph /var/www/customer/tmp/ZtIKQOJZFf.png --alt-autoscale 
--rigid --start now-3600 --end now --width 800 --height 350 
-c BACK#EEEEEE00 -c SHADEA#EEEEEE00 -c SHADEB#EEEEEE00 -c FONT#000000 
-c GRID#a5a5a5 -c MGRID#FF9999 -c FRAME#5e5e5e -c ARROW#5e5e5e -R normal 
--font LEGEND:8:'DejaVuSansMono' --font AXIS:7:'DejaVuSansMono' --font-render-mode normal 
-E COMMENT:'Bits/s Last Avg Max \n' 
DEF:sfptxpower=/var/www/customer/rrd/sfpdbm.rrd:SFPTXPOWER:AVERAGE 
DEF:sfprxpower=/var/www/customer/rrd/sfpdbm.rrd:SFPRXPOWER:AVERAGE 
DEF:sfptxpower_max=/var/www/customer/rrd/sfpdbm.rrd:SFPTXPOWER:MAX 
DEF:sfprxpower_max=/var/www/customer/rrd/sfpdbm.rrd:SFPRXPOWER:MAX 
LINE1.25:sfptxpower#000099:'tx ' GPRINT:sfptxpower:LAST:%6.2lf%s\g 
GPRINT:sfptxpower:AVERAGE:%6.2lf%s\g GPRINT:sfptxpower_max:MAX:%6.2lf%s\g 
COMMENT:'\n' LINE1.25:sfprxpower#B80000:'rx ' 
GPRINT:sfprxpower:LAST:%6.2lf%s\g GPRINT:sfprxpower:AVERAGE:%6.2lf%s\g 
GPRINT:sfprxpower_max:MAX:%6.2lf%s\g COMMENT:'\n'

which draws a graph just how it is supposed to be. However, the graph that comes out of it is not very readable as both tx and rx values make up the border of the graph:

My question therefor is: Is it possible to add some sort of margin (like a percentage (%)?) to the X-axis so that both lines can be easily seen on the graph?

Answer 1

RRDTool graph has four different scaling modes you can select via options: autoscale (the default), alt-autoscale, specified-expandable, and specified-rigid.

Autoscale - this scales the graph to fit the data, using the default algorythm. You choose this using the --autoscale option (or by omitting the other scaling options). This will try to make the Y-axis range limited by common ranges -- in your case, probably 0 to -5. Sometimes it works well, sometimes it doesnt.

Alt-Autoscale - this is like autoscale, but clings closely to the actual data max and min. You choose this with --alt-autoscale and it is what you are currently using.

Specified, expandable - This lets you specify a max/min for the Y axis, but they are expanded out if the data are outside this range. You choose this by specifying --upper-limit and/or --lower-limit but NOT --rigid. In your case, if you give an upper limit of -2 and a lower limit of -4 it would look good, and the graph range would be expanded if your data go to -5.

Specified, rigid - This is like above, but the limits are fixed where you specify them. If the data go outside this range, then the line is not displayed. You specify this by using --rigid when giving an upper or lower bound.

Note that with the Specified types, you can specify only one end of the range so as to get a specified type at one end and continue to use an autoscale type for the other.

From this, I would suggest that you remove the --rigid and --alt-autoscale options, and instead specify --upper-limit -2 and --lower-limit -4 to display your data more neatly. If they leave this range then you will continue to get a graph as currently - whether this works or not depends on the nature of the data and how much they normally can vary.