Mastering the Gnuplot Key: Customizing Plot Legends and More

Gnuplot is an incredibly powerful tool for visualizing data and generating high-quality plots. One of its standout features is the ability to include a "key" (or legend) that helps viewers interpret the meaning behind the plotted data. The key allows users to label different data series or plot elements, making it easier to understand the context of the graph. In this article, we’ll take an in-depth look at the Gnuplot key: what it is, how it works, and how to customize it to fit your needs. Let’s dive in!

What is the Gnuplot Key?

The Gnuplot key, commonly referred to as a legend, is a graphical element that labels the different data series in a plot. It serves to distinguish between various plotted functions, data points, or lines in the graph, making it easier for the viewer to understand what each element represents. In Gnuplot, the key can be customized in terms of position, font, color, and more.

Gnuplot plots often have multiple datasets or functions, especially in cases where you want to compare trends or display different conditions on the same graph. The key helps to provide context by associating each dataset with a label, typically appearing as text next to a corresponding symbol or line.

Basic Usage of the Gnuplot Key

To enable a key in a Gnuplot plot, you simply use the set key command. By default, Gnuplot places the key in the upper-right corner of the plot. For example, to create a basic plot with a key, you could use the following command:

    set key
    plot sin(x), cos(x)

In this case, the plot will display the sine and cosine functions, each labeled in the key. The key will automatically label the functions based on their respective data series.

Customizing the Gnuplot Key

While the default key placement and appearance might work for simple plots, you’ll likely want to customize the key to better suit your needs. Gnuplot provides several options for adjusting the key’s position, style, and appearance.

1. Changing the Position of the Key

The position of the key can be customized using the set key command followed by the location keyword. Gnuplot allows you to position the key in any of the plot’s corners or along the edges. Common positions include:

    set key top left
    set key bottom right
    set key center

You can also specify a specific position by giving coordinates, such as:

    set key at 0.5, 0.9

This places the key at the coordinates (0.5, 0.9) on the graph, allowing you to fine-tune the key’s placement.

2. Changing the Key Title and Labels

To give your key more context, you can modify the labels that appear next to each data series. By default, Gnuplot uses the title of the plot element as the label for the key. However, you can specify custom labels for each data series using the title option within the plot command. Here’s an example where we provide custom titles for the sine and cosine functions:

 plot sin(x) title "Sine Wave", cos(x) title "Cosine Wave" 

In this example, the key will display "Sine Wave" and "Cosine Wave" instead of just "sin(x)" and "cos(x)", providing clearer context for the viewer. This is especially useful when dealing with more complex plots with multiple datasets or functions.

3. Modifying the Key’s Font and Appearance

Gnuplot allows you to customize the font used for the key, as well as the size and style. You can change the font type, size, and whether it appears bold, italicized, etc. For instance, to change the font to Arial and set the size to 14, you can use the following command:

 set key font "Arial,14" 

Additionally, you can adjust the color of the key text using the set key textcolor command:

 set key textcolor rgb "blue" 

These modifications help you style the key to match the overall aesthetic or clarity of your plot.

4. Adjusting the Key Box and Border

The key’s box can also be customized. By default, the key has a border around it, but you can remove the border or adjust its style. Here’s how to remove the key box:

 set key box no 

If you want to keep the box but modify its appearance, you can adjust the line type, color, and width. For example:

 set key box linewidth 2 linetype 1 

This command sets the key’s border to be 2 pixels wide with a solid line. You can also use other line types and colors depending on your preferences.

5. Rotating the Key

In some cases, especially with larger plots, you may want to rotate the key to improve readability. This can be done using the set key rotate command, which allows you to rotate the key’s text. Here’s how to rotate the key by 45 degrees:

 set key rotate by 45 

This can help when space is limited or if you want to adjust the orientation to make the plot more legible.

Example: Customizing a Gnuplot Plot with a Key

Let’s put everything together with a full example. Suppose you have two datasets: one representing the temperature over time and another representing the humidity over time. You can plot them with a customized key as follows:

 set title "Temperature and Humidity Over Time" set xlabel "Time (days)" set ylabel "Temperature (°C)" set y2label "Humidity (%)" set y2tics set key top left font "Arial,12" textcolor rgb "darkred" box linewidth 2 linetype 1 plot "temperature_data.dat" using 1:2 with lines title "Temperature",  "humidity_data.dat" using 1:2 with lines title "Humidity" axis x1y2 

This script does the following:

• set title – sets the title of the plot.
• set xlabel and set ylabel – label the x-axis and primary y-axis.
• set y2label – labels the secondary y-axis.
• set y2tics – enables ticks for the secondary y-axis.
• set key top left font "Arial,12" textcolor rgb "darkred" box linewidth 2 linetype 1 – customizes the key’s position, font, text color, border, and line type.
• plot – creates the plot with two datasets, each labeled in the key.

With this customization, you’ll get a plot with two datasets, each labeled clearly in the key with customized fonts, colors, and styles. The key will appear in the top-left corner, making it easy to distinguish between the temperature and humidity data.

Conclusion

Customizing the Gnuplot key is a powerful way to make your plots more readable and visually appealing. Whether you're adjusting the position, font, color, or adding a box around the key, Gnuplot provides a wide range of options to tailor the key to your specific needs. By using the key effectively, you can ensure that your data is presented clearly and that viewers can easily interpret the graph.

Experiment with these options to find the best configuration for your plots, and soon you’ll be creating polished, professional-quality graphs that are both informative and visually striking.