Mastering Gnuplot: A Versatile Graph Plotter (with examples)

Gnuplot is a powerful, portable command-line driven graph plotter widely used for creating 2D and 3D graphics of mathematical functions and data. It provides numerous output formats, including screen display (using X11, AquaTerm, or others), as well as file outputs in formats like PNG, JPEG, SVG, EPS, PDF, and more. It helps in visualizing complex data and functions, making data analysis and presentation much simpler and more effective.

Use case 1: Start the interactive graph plotting shell

Code:

gnuplot

Motivation:

The interactive shell of Gnuplot offers a dynamic and efficient workspace for users to explore data plots and experiment with different plotting commands and functions interactively. This is particularly useful during the initial phases of data analysis where you want to try out different plots and adjust parameters like axis scales, labels, and other settings interactively.

Explanation:

• gnuplot: This command initiates the Gnuplot interactive shell interface, allowing users to enter commands interactively for real-time graph plotting. Once in the shell, users can define functions, set terminal output types, plot equations, and adjust styles with immediate feedback.

Example output:

Upon entering the command, a prompt appears which looks like:

gnuplot>

This indicates that the Gnuplot shell is active and ready to accept commands for plotting.

Use case 2: Plot the graph for the specified graph definition file

Code:

gnuplot path/to/definition.plt

Motivation:

Often, users need to automate plotting tasks using pre-defined scripts to handle complex plotting commands efficiently. This is where the ability to run graph definition files (scripts) becomes indispensable. By defining these parameters once in a script file, you ensure consistency and save time, especially for repetitive tasks.

Explanation:

• gnuplot: Launches the Gnuplot program in non-interactive mode where script execution takes place.
• path/to/definition.plt: This specifies the file path to the Gnuplot script (or definition file), which contains all the necessary commands for plotting. Typical contents of such scripts include data file loading, plot styles, set commands for axes, and plot definitions.

Example output:

When executed, the command processes the definition script and produces a graph according to the instructions provided within definition.plt. The output can be visualized on-screen or saved to an output file depending on the instructions in the script.

Use case 3: Set the output format by executing a command before loading the definition file

Code:

gnuplot -e "set output 'path/to/filename.png' size 1024,768" path/to/definition.plt

Motivation:

Flexibility in output formats is crucial for integrating graph outputs into various reports and documents. By specifying the output settings at runtime, users can easily adjust the resolution and dimensions of the plot, which is incredibly handy when different quality or size requirements are needed without altering the original script.

Explanation:

• -e "set output 'path/to/filename.png' size 1024,768": This flag allows you to execute setup commands directly from the command line before running your script. Here, it:
  • set output 'path/to/filename.png': Directs Gnuplot to write the graph to a PNG file instead of displaying it on screen.
  • size 1024,768: Determines the size of the outputted image, setting it to a resolution of 1024 by 768 pixels.
• path/to/definition.plt: This path points to the script file containing plotting commands, ensuring compliance with the predefined plot instructions.

Example output:

With this command, a PNG image named filename.png is generated with the dimensions specified. This file is stored at the given location for further use.

Use case 4: Persist the graph plot preview window after gnuplot exits

Code:

gnuplot --persist path/to/definition.plt

Motivation:

When analyzing data, it’s often beneficial to keep the plotted graph visible after Gnuplot exits to allow users to compare results, present findings, or simply revisit the data for manual inspection without re-running the script. The --persist option ensures that the graph remains displayed in a separate window even after the original Gnuplot command has completed execution.

Explanation:

• --persist: This option allows the plot window to remain open after the script execution has terminated. This function is invaluable for prolonged analysis and comparison against other data or variables without the need to re-plot.
• path/to/definition.plt: Refers to the file containing plotting instructions, which Gnuplot will execute while displaying the plot as a persistent window.

Example output:

After execution, the plot is rendered and remains accessible in a graphical window, allowing users to close it manually when the plot is no longer required for viewing, ensuring complete control over the display duration.

Conclusion:

Gnuplot is a versatile and indispensable tool for those requiring high-quality graphical representations in varied formats. Each of the illustrated use cases demonstrates practical functionalities that simplify and enhance the data visualization experience, catering to both quick exploratory analysis and detailed presentation needs. Through this capability, Gnuplot proves to be an essential part of any data analyst’s toolkit.