Gnuplot Y2 Axis: How to Master Dual Y-Axis Plots

When it comes to plotting data, Gnuplot is one of the most versatile tools available. One of its most powerful features is the ability to use multiple y-axes, which is especially useful when you have two sets of data with different scales. The y2 axis feature in Gnuplot allows you to create plots that display data on both the primary y-axis and a secondary y-axis, giving you greater flexibility in how you present complex data. In this article, we will explore the gnuplot y2 axis in detail, providing examples and tips on how to make the most of this feature in your plots.

What is the Gnuplot Y2 Axis?

The y2 axis in Gnuplot refers to the secondary y-axis that can be added to a plot, allowing you to display two sets of data that may have different ranges or units. This feature is particularly useful when comparing two different variables on the same graph but with different scales. For example, you might want to plot temperature on the primary y-axis and humidity on the secondary y-axis, with each having its own scale.

The y2 axis enables you to overlay two sets of data on the same plot, making it easier to visualize the relationship between the two variables. The key advantage is that it allows you to keep both datasets visible without distorting the scales of either one, giving you a clearer picture of how they interact. By using the y2 axis, you can create more informative and readable plots.

Basic Syntax for Using the Y2 Axis

To use the y2 axis in Gnuplot, you need to specify the use of the secondary axis in your plot command. This is done by adding the axes option to the plot command, indicating that a second y-axis should be used. Here’s the basic syntax:

plot 'datafile' using 1:2 axes x1y1 title 'Primary Data', 
     'datafile' using 1:3 axes x1y2 title 'Secondary Data'

In this example, the first dataset is plotted using the primary y-axis (axes x1y1), and the second dataset is plotted using the secondary y-axis (axes x1y2). The title option specifies the label that will appear in the legend for each dataset.

Practical Examples of Using the Y2 Axis

Let’s dive into some practical examples to see how the y2 axis can be used in real-life scenarios. These examples will help you better understand how to implement this feature and how to interpret the resulting plots.

Example 1: Plotting Temperature and Humidity

Imagine you have a dataset with temperature and humidity measurements taken over time, and you want to plot both variables on the same graph. Since temperature and humidity have different units and ranges, it would be useful to use the secondary y-axis. Here’s how you could do that:

# Data format: time, temperature, humidity
plot 'data.txt' using 1:2 axes x1y1 title 'Temperature (°C)', 
     'data.txt' using 1:3 axes x1y2 title 'Humidity (%)'

In this case, the temperature is plotted on the primary y-axis (x1y1), while the humidity is plotted on the secondary y-axis (x1y2). This allows both sets of data to be displayed clearly without distorting the scales.

Example 2: Comparing Stock Prices and Trading Volume

Another common use case for the y2 axis is when you want to compare stock prices and trading volumes on the same plot. Stock prices are typically measured in dollars, while trading volume is measured in shares. Since these two variables have very different scales, it’s useful to display them on two separate y-axes:

# Data format: date, stock price, trading volume
plot 'stock_data.txt' using 1:2 axes x1y1 title 'Stock Price (USD)', 
     'stock_data.txt' using 1:3 axes x1y2 title 'Volume (shares)'

Here, the stock price is plotted using the primary y-axis, and the trading volume is plotted using the secondary y-axis. The result is a clear comparison between the two variables over time, without the need to scale one variable artificially.

Customizing the Appearance of the Y2 Axis

One of the great advantages of using the y2 axis in Gnuplot is the ability to customize the appearance of the secondary axis. You can modify the axis labels, scale, and even the color of the data to make your plot more visually appealing and easier to interpret. Below are some customization options you can use:

Customizing the Label of the Y2 Axis

You can change the label of the secondary y-axis to better describe the data being plotted. For example, if you’re plotting the humidity on the secondary axis, you can set the label to something more informative:

set ylabel 'Humidity (%)' axis x1y2

This command sets the label of the secondary y-axis to "Humidity (%)", making it clear what data is represented on that axis.

Changing the Scale of the Y2 Axis

If the data on the secondary axis has a different scale or range than the primary axis, you can adjust the scale of the y2 axis to fit the data. This can be done by specifying the range of the secondary axis:

set yrange [0:100] axis x1y2

In this example, the range of the secondary y-axis is set from 0 to 100, which is useful if your humidity data is constrained within this range.

Changing the Color of the Y2 Axis Data

To make the secondary dataset stand out, you can change the color of the plot for the y2 axis:

plot 'data.txt' using 1:2 axes x1y1 title 'Primary Data', 
     'data.txt' using 1:3 axes x1y2 title 'Secondary Data' lc rgb 'red'

In this case, the data plotted on the secondary y-axis will appear in red, making it easier to distinguish from the primary dataset.

Tips and Best Practices for Using the Y2 Axis

When using the y2 axis in Gnuplot, there are a few best practices to keep in mind to ensure that your plots are clear and easy to understand:

• Label everything clearly: Make sure to label both y-axes and provide a clear legend. This will help viewers understand which data corresponds to which axis.
• Avoid clutter: Don’t overload your plot with too many datasets. Using a secondary y-axis is useful, but too much information on a single plot can become overwhelming.
• Use contrasting colors: If you're plotting multiple datasets on the same graph, use contrasting colors to make it easy to differentiate between the datasets on the primary and secondary y-axes.
• Check the scales: Always ensure that the scales on both y-axes are appropriate for the data being plotted. If the ranges are too wide or too narrow, it can lead to misinterpretation of the data.

Conclusion

The y2 axis feature in Gnuplot is a powerful tool for visualizing data that has different units or scales. By using the secondary y-axis, you can plot multiple datasets on the same graph without distorting the scales, making it easier to compare and interpret the data. In this article, we’ve covered the basics of using the y2 axis, practical examples, customization options, and best practices. Armed with this knowledge, you can create more informative, clear, and visually appealing plots in Gnuplot.