Introduction

This section covers some of the more advanced uses of color in a RRDTool graph.  These techniques can not only help in providing an additional bit of polish to an otherwise ordinary graph, but can also be useful in clarifying interpretation and providing additional information.

Despite the availability of the techniques outlined below, it is still essential that proper color conventions and patterns be observed.  For example, red typically denotes “hot” in the context of temperature or “severe” in a notification/aberration detection.  Using red to denote a cool temperature on a temperature graph or a “normal” operating condition is very likely to lead to viewer confusion.  For more information, please be sure to consult a good reference on color theory.

Examples

Example 1

This is a simple example of a “stock” graph.  It uses no special color treatments but it is still able to clearly convey the necessary information.  In this case, it simply presents a graph of the system temperature.

rrdtool graph "Example 1 Colors.png" \
--start "end-48 hours" --end "12am Nov 1, 2009" \
--imgformat PNG --width 500 --height 120 \
--title "Example 1" \
--vertical-label "Temperature" \
DEF:temp=sysinfo.rrd:temperature:AVERAGE \
AREA:temp#FF0000

Example 2

In this example, the use of an “alpha channel” in the color specification is introduced.  The easiest way to think of an alpha channel is as a transparency measure.  A alpha channel of “FF” would be completely opaque, while an alpha channel of “00″ would be completely transparent.  The alpha channel is specified in hexadecimal form at the end of the RGB color value.  In the example below, the color value of “FF000044″ has specified an alpha channel value of “44″.  If no alpha channel is specified, then a default value of “FF” (fully opaque) is used.

rrdtool graph "Example 2 Colors.png" \
--start "end-48 hours" --end "12am Nov 1, 2009" \
--imgformat PNG --width 500 --height 120 \
--title "Example 2" \
--vertical-label "Temperature" \
DEF:temp=sysinfo.rrd:temperature:AVERAGE \
AREA:temp#FF000044

Example 3

This example illustrates the use of the “layer cake” effect.  This technique can help to provide additional context to a graph, as the colored layers can help clearly delineate when a system is operating within tolerances or not.  Thus, the health of the system can be determined at a glance and is not dependent on the viewer being intimate with the operational thresholds.  This example breaks down the temperature readings into four layers (cold, cool, warm, and hot) of 50 degrees each, but it would be a trivial extension to increase/decrease the number of layers.

The CDEF for the middle layers can be somewhat intimidating for those who are not experts in Reverse Polish Notation.  In this example, each layer relies on being stacked and so the appropriate calculation is determining the portion of the temperature (if any) that makes up the layer.  The following breakdown may help make it more palatable:

cool=temp,50,GT,temp,100,GT,50,temp,50,-,IF,UNKN,IF
if (temp > 50) then
  if (temp > 100) then
    cool = 50
  else
    cool = temp - 50
else
  cool = UNKN

As each layer is a maximum of 50 degrees, the trick is to determine how much (if any) of a layer falls within the designation.  If the actual temperature exceeds that of the layer, then simply use the maximum value (50).  If the temperature falls within the layer, then the value should be the temperature less the total of any previous bands.  If the temperature is less than the minimal temperature for this layer, then simply return the “unknown” value to prevent any graphing.

rrdtool graph "Example 3 Colors.png" \
--start "end-48 hours" --end "12am Dec 5, 2009" \
--imgformat PNG --width 500 --height 120 \
--title "Example 3" \
--vertical-label "Temperature" \
--lower-limit 0 --rigid \
DEF:temp=sysinfo.rrd:temperature:AVERAGE \
CDEF:cold=temp,50,LE,temp,50,IF \
CDEF:cool=temp,50,GT,temp,100,GT,50,temp,50,-,IF,UNKN,IF \
CDEF:warm=temp,100,GT,temp,150,GT,50,temp,100,-,IF,UNKN,IF \
CDEF:hot=temp,150,GT,temp,150,-,UNKN,IF \
AREA:cold#0000FFAA:cold:STACK \
AREA:cool#0000FF44:cool:STACK \
AREA:warm#FF000044:warm:STACK \
AREA:hot#FF0000AA:hot:STACK

Example 4

There are several techniques for “feathering” the colors in a graph as shown in this example.  The technique illustrated in this example is suitable for a representing a single color palette with the gradient lightest at the top and darkest at the bottom.  It is achieved by simply overlaying the graph with the same color selection at selected proportions and relying on the alpha channel to “build up” as the layers overlap.  Care should be made when using this technique not too make the top layers so translucent they become difficult to discern.

This example simply maps the set of data values into sets for 1/4, 1/2 and 3/4 values and then overlays the original value graph.  Additional looks can also be achieved through the use of alternative data transformation maps/ratios.

rrdtool graph "Example 4 Colors.png" \
--start "end-48 hours" --end "12am Dec 5, 2009" \
--imgformat PNG --width 500 --height 120 \
--title "Example 4" \
--vertical-label "Temperature" \
--lower-limit 0 --rigid \
DEF:temp=sysinfo.rrd:temperature:AVERAGE \
CDEF:tier1=temp,4,/ \
CDEF:tier2=temp,2,/ \
CDEF:tier3=temp,4,/,3,* \
AREA:temp#FF000022: \
AREA:tier3#FF000022: \
AREA:tier2#FF000022: \
AREA:tier1#FF000022:

Example 5

This example illustrates another method of “feathering” the colors in the graph.  In this case, the color gradient is lightest at the bottom and darkest at the top.  In order to achieve this, the value is simply divided up and then each layer is stacked on top of the other while steadily increasing the alpha channel value.

rrdtool graph "Example 5 Colors.png" \
--start "end-48 hours" --end "12am Dec 5, 2009" \
--imgformat PNG --width 500 --height 120 \
--title "Example 5" \
--vertical-label "Temperature" \
--lower-limit 0 --rigid \
DEF:temp=sysinfo.rrd:temperature:AVERAGE \
CDEF:tier=temp,4,/ \
AREA:tier#FF000022::STACK \
AREA:tier#FF000044::STACK \
AREA:tier#FF000066::STACK \
AREA:tier#FF000088::STACK

Example 6

This example illustrates the use of highlights to clearly delineate the borders between stacked area graphs.  It allows the use of a softer color palette without having to resort to a clashing color scheme to define the borders.

The highlight lines should be specified after all the area graphs have been declared.  Each highlight should be specified in the same order as its corresponding area graph in order to ensure the proper color is “on top” should the data sets have any overlap.  It is typically easiest to maintain the same color scheme by using the same RGB value as the area graph but specifying high alpha channel value.

rrdtool graph "Example 6 Colors.png" \
--start "end-48 hours" --end "12am Jan 15, 2009" \
--imgformat PNG --width 500 --height 120 \
--title "Example 6" \
--vertical-label "Bytes" \
--lower-limit 0 --rigid \
DEF:disk1=sysinfo.rrd:disk_used:AVERAGE \
DEF:disk2=sysinfo.rrd:disk2_used:AVERAGE \
AREA:disk1#0000FF22:: \
AREA:disk2#00F00022::STACK \
LINE1:disk1#0000FFAA:"Disk 1" \
LINE1:disk2#00F000AA:"Disk 2":STACK