Real-time plots in 20 lines
Simple Line Chart
A line chart presentation can be created from a data file (GPU.dat
), as well:
# GPU.dat - a time stamp with two data points 18:48:30 51.0 49.0 18:48:40 50.5 49.5 18:48:45 51.5 49.0 18:48:50 50.0 50.5 18:48:55 50.5 49.5
The interactive Gnuplot commands to show a line chart of this data are shown in Listing 2. This Gnuplot script requires a few extra lines: The plot needs to know that the x-axis is time data, and it needs to know the format of the time data and the x labels.
Listing 2
Line Chart from a File
$ gnuplot gnuplot> # plot 2 variables in the file GPU.dat gnuplot> # gnuplot> set xdata time gnuplot> set timefmt "%H:%M:%S" gnuplot> set format x "%H:%M:%S" gnuplot> plot "GPU.dat" using 1:2 with line title "GPU temp" , "GPU.dat" using 1:3:3 with line title "CPU temp"
Multiple data points can be plotted at the same time (Figure 4). The using
argument tells Gnuplot how to reference the <x>:<y>
columns in the data file. (If the data file had a third column of data points, the using
reference to get the last column of data would be 1:4:4
).
Real-Time Line Chart
Linux has a lot of useful command-line troubleshooting tools, such as iostat
, vmstat
, and top
, to name just a few. For the line chart example, I use the sensors
utility [3] to get the fan speed and CPU temperature of my Linux server. The sensors
command returns a number of lines of information.
$ sensors dell_smm-virtual-0 Adapter: Virtual device Processor Fan: 2676 RPM CPU: +47.0~ZâC Ambient: +38.0âC SODIMM: +37.0âC ...
With some Bash and Awk commands, you can get just the fan speed and CPU temperature (Listing 3).
Listing 3
Parsing Data
$ sensors | grep RPM Processor Fan: 2685 RPM $ sensors | grep RPM | awk '{print $3}' 2685 $ sensors | grep CPU CPU: +48.0âC $ sensors | grep CPU | awk '{print $2}' +48.0âC $ sensors | grep CPU | awk '{print substr($2,2,4)}' 48.0
Awk supports a systime()
call to return the present date/time, and a strftime()
call to customize the presentation. (Note: You might have to install gawk
– sudo apt-get install gawk
– on the Raspberry Pi to get this added functionality.)
Once the measurements have been parsed, the next step is to format the sensor output with a timestamp:
$ sensors | grep RPM |awk '{print strftime("%H:%M:%S ",systime()) $3}' 10:26:46 2685 $sensors | grep CPU |awk '{print strftime(\"%H:%M:%S \",systime()) substr($2,2,4)}' 10:27:46 48.0
After a time and value string have been generated, you can create a Gnuplot script, line_fan_cpu.txt
, to show real-time data (Listing 4). To make the script a little easier, I create two data files, fan.dat
and cpu.dat
.
Listing 4
Dynamic Line Chart Script
01 # Create a Plot or User and System CPU Usage, update every 5 seconds 02 # 03 set title "GnuPlot - Fan Speed and CPU Temperature" 04 set yrange [2650:2700] 05 set ylabel "Fan Speed" 06 set y2range [43:49] 07 set y2label "CPU Temp (C)" 08 set y2tics 09 set xdata time 10 set timefmt "%H:%M:%S" 11 set format x "%H:%M:%S" 12 13 system "sensors | grep RPM | awk '{print strftime(\"%H:%M:%S \", systime()) $3}' > fan.dat" 14 system "sensors | grep CPU | awk '{print strftime(\"%H:%M:%S \",systime()) substr($2,2,4)}' > cpu.dat" 15 16 plot "fan.dat" using 1:2 with lines axes x1y1 title "fan speed (RPM)", "cpu.dat" using 1:2 with lines axes x1y2 title "CPU Temp (C)" 17 while (1) { 18 pause 5 19 system "sensors | grep RPM | awk '{print strftime(\"%H:%M:%S \", systime()) $3}' >> fan.dat" 20 system "sensors | grep CPU | awk '{print strftime(\"%H:%M:%S \", systime()) substr($2,2,4)}' >> cpu.dat" 21 replot 22 }
The plot accounts for different scale ranges with y2range
and y2label
definitions. The final addition is to include an axis (x1y2
or x1y2
) to each plot point that lines up the data values to the right or left y-axis.
The complete Gnuplot script to show fan speed and CPU temperature is only 20 lines of code! The command
$ gnuplot -persist line_fan_cpu.txt
runs this script (Figure 5).
Final Comments
I won't give up using plotting packages like Matplotlib or ggplot, but I was very impressed with how easy it was to create real-time plots with Gnuplot.
Manipulating the Bash/Awk script can be a little complex, but it's incredibly useful to be able to use output from almost any command-line utility in Gnuplot.
Gnuplot can plot a large number of data points, but it makes sense to do a tail
command to create a sliding view of the latest information.
Infos
- Gnuplot documentation: http://www.gnuplot.info/
- gpio command-line utility: http://wiringpi.com/the-gpio-utility/
- sensors command-line utility: https://wiki.archlinux.org/index.php/Lm_sensors
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