Shade a Range on the Y-Axis

Question

When graphing with RRDTool, I have added two VRULEs. I'm graphing temperature, and I would like to have the area between them shaded in on the graph. Does RRDTool have any option like this? I've perused https://oss.oetiker.ch/rrdtool/doc/rrdgraph_graph.en.html and can't find anything.

Answer 1

Yes, this is possible, though you need to do a little trick to achieve it. The Routers2 software uses this trick to highlight Working Hours during the time window.

In order to do this, you create a dummy CDEF that returns a time series that is +INF during the highlighted window, and 0 elsewhere. Then, you graph this as an AREA, either before your lines, or after (with transparentcy).

The trick is that the CDEF has to be based on a DEF, though it can throw away the data and then calculate based on the TIME value instead.

For example (simplified for clarity);

rrdtool graph output.png \
    DEF:x=file.rrd:ds:AVERAGE \
    CDEF:bg=x,POP,0,INF,TIME,$mintime,$maxtime,LIMIT,UN,IF \
    AREA:bg#ff808080 \
    LINE:x#00ff00:Value \
    VRULE:$mintime#ff8080 \
    VRULE:$maxtime#ff8080

This will draw a line (in green) for the data; behind it is a seethrough pink highlight between times $mintime and $maxtime, and at these two times, a red VRULE is overlaid.

Note how the CDEF throws away the initial value of x, and instead uses LIMIT to tell if the sample time is in the range, subsequently using IF to give a value of either 0 or INF depending on the result. This is then used for the AREA that forms the highlight.

Instead of using a set $mintime and $maxtime, you could instead use the value of LTIME (which takes into account your local timezone offset) to calculate the actual time of day, and highlight between 9am and 5pm M-F. This is the trick used by Routers2 to enable it to highlight multiple areas for working hours.