Real-time plots in 20 lines
The Plot Twists

© Lead Image © Volodymyr Horbovyy, 123rf.com
Use Gnuplot with command-line utilities.
Some excellent charting and plotting packages can be found, but if you're like me, you sometimes just want to do a quick dynamic test plot without a lot of custom setup. Gnuplot is a command-line charting utility that has been around for a while, and I was amazed how easy it was to get up and running. In only 20 lines of scripting code, I was able to create real-time line and bar charts.
In this article, I introduce Gnuplot with two dynamic examples: The first shows the status of Raspberry Pi I/O pins, and the second is a line chart of CPU diagnostics.
Getting Started
Gnuplot [1] can be installed on Linux, Windows, and macOS. To install Gnuplot on Ubuntu, enter:
sudo apt-get install gnuplot
Gnuplot is typically run as a command-line utility, but it can also be run manually, with the charting instructions and data values inserted inline. To plot four sets of data points in a line chart, you could enter:
$ gnuplot gnuplot> $Mydata << EOD # Now enter some data 2 1 3 1.5 4 2.1 5 3.3 EOD gnuplot> plot $Mydata with line
Data block names must begin with a $
character, which distinguishes them from other types of persistent variables. The end-of-data delimiter (EOD
here) can be any sequence of characters. For this example, the plot command creates a line chart from the $Mydata
variable (Figure 1).

Static Bar Chart
For a simple Gnuplot bar chart, you could plot the real-time status of Raspberry Pi general purpose input/output (GPIO) pins. A static bar chart presentation can be created with a data file (called gpio.dat
here):
# gpio.dat - data file for GPIO pin values # column1 = chart position, column2 = heading, column3 = value 0 GPIO2 0 1 GPIO3 1 2 GPIO4 1 # ...
To plot a bar chart (Figure 2), the fill style and bar width need to be defined. The using 1:3:xtic(2)
argument, shown in the next code block, configures the first column in the data file as the x position, the third column as the y value, and the second column as the x-axis labels. Use the interactive commands
$ gnuplot gnuplot> set style fill solid gnuplot> set boxwidth 0.5 gnuplot> plot "gpio.dat" using 1:3:xtic(2)with boxes title ""
to plot the file.

Real-Time Bar Chart
The previous example used a manually created gpio.dat
data file. The current status of GPIO pins can be found with the gpio
command-line utility [2]. For example, to get the status of GPIO pin 9, enter:
gpio read 9
By adding some Bash and an Awk script, you can create a gpio.dat
file:
$ gpio read 9 1 $ gpio read 9 | awk '{ print "9 GPIO9 " $1 }' 9 GPIO9 1 $ gpio read 9 | awk '{ print "9 GPIO9 " $1 }' >gpio.dat $ cat gpio.dat 9 GPIO9 1
To make a dynamic bar chart, create the gpio_bars.txt
Gnuplot script shown in Listing 1. The Gnuplot scripting language is quite powerful and supports a wide range of functions and control statements.
Listing 1
Dynamic Bar Chart
01 # Create a dynamic bar chart that reads GPIO pins every 5 seconds 02 # 03 set title "PI GPIO Data" 04 set boxwidth 0.5 05 set style fill solid 06 07 # Create a dummy file to get started without errors 08 system "echo '0 GPIO2 1' > gpio.dat" 09 10 plot "gpio.dat" using 1:3:xtic(2) with boxes title "" 11 12 while (1) { # make a new 'gpio.dat' every cycle with fresh data 13 system "echo '' > gpio.dat" 14 do for [i=2:29] { 15 j = i-2 # put first GPIO pin at position 0 16 system "gpio read " .i. " | awk '{ print \"" . j . " GPIO" . i . " \" $1 }' >> gpio.dat 17 } 18 replot 19 pause 5 20 }
Rather than manually adding lines for each GPIO pin status, a for
loop can iterate from pins 2 to 29 (lines 14-17). A system
command runs the GPIO utility and Bash commands (line 16). To refresh the data, use the replot
and pause
commands (lines 18 and 19), and enter
gnuplot -persist gpio_bars.txt
to run the script (Figure 3).

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