Mastering the gnuplot Y2 Axis: A Complete Guide with Examples

If you've ever found yourself dealing with multiple datasets that have different scales, you'll know that plotting them on the same graph can become quite challenging. Fortunately, gnuplot, a powerful data visualization tool, offers a solution: the y2 axis. In this article, we'll walk through the use of the gnuplot y2 axis, how to create plots with dual y-axes, and provide practical examples that demonstrate its power. Get ready to take your plotting skills to the next level!

What is the gnuplot Y2 Axis?

The y2 axis in gnuplot allows you to plot data on a secondary vertical axis, which can be useful when you are comparing two datasets with different ranges. For example, if one dataset ranges from 0 to 100 and another from 0 to 1000, plotting both on the same axis can make the graph hard to read. By using the y2 axis, you can plot one dataset on the primary y-axis and the other on the secondary y2 axis, allowing for a clearer, more interpretable graph.

The y2 axis in gnuplot works by associating different data sets with the right-side axis. This way, you can plot and visualize two different types of data on the same x-axis, but on different y-axes. This is particularly useful for scientific plots, financial charts, or any situation where you want to compare two variables that are on vastly different scales.

How to Enable the Y2 Axis in gnuplot

Before diving into examples, let's take a look at the basics of enabling the y2 axis in gnuplot. In gnuplot, enabling the secondary y-axis is straightforward. You need to use the set y2tics command to tell gnuplot that you want to use the y2 axis for plotting, and you can configure the tick marks on the secondary axis with set ytics.

Here’s an example of how to enable the secondary y-axis:

set y2tics
set ytics
plot "datafile1.dat" using 1:2 with lines, "datafile2.dat" using 1:3 axes x1y2 with lines

In this example, using 1:2 refers to the first file where the x-axis corresponds to the first column and the y-axis corresponds to the second column. axes x1y2 specifies that the second data set should be plotted on the secondary y-axis (y2), while the first dataset uses the primary y-axis (y1).

Plotting Two Data Sets on Different Y-Axes

Now that you know how to enable the y2 axis, let’s see a full example of plotting two datasets with different scales using the primary and secondary y-axes. This is where the y2 axis truly shines!

Consider the following two datasets:

• datafile1.dat – Contains temperature data (in degrees Celsius) over time.
• datafile2.dat – Contains pressure data (in pascals) over the same time period.

We want to plot temperature on the primary y-axis and pressure on the secondary y2 axis. The gnuplot script for this would look something like this:

set xlabel "Time (s)"
set ylabel "Temperature (°C)"
set y2label "Pressure (Pa)"
set y2tics
plot "datafile1.dat" using 1:2 with lines title "Temperature", 
     "datafile2.dat" using 1:3 axes x1y2 with lines title "Pressure"

In this script:

• We use set xlabel and set ylabel to label the x and y axes for the primary axis.
• set y2label adds a label to the secondary y-axis (y2).
• set y2tics enables the tick marks on the y2 axis.
• The axes x1y2 command ensures that the second dataset is plotted against the secondary y-axis.

This would result in a graph where temperature is plotted against the primary y-axis (on the left), and pressure is plotted against the secondary y-axis (on the right). The x-axis represents time, which is shared between both datasets.

Customizing the Y2 Axis

One of the most powerful features of gnuplot is its flexibility. You can easily customize the appearance of the y2 axis to better suit your needs. For example, you can adjust the tick marks, the range, and the format of the labels on the secondary y-axis.

Let’s look at some common customizations for the y2 axis:

# Adjusting the range of the y2 axis
set yrange [0:1000]
set y2range [0:500000]

# Customizing tick marks on the y2 axis
set y2tics 100000, 100000, 500000

# Changing the font of the y2 axis label
set y2label "Pressure (Pa)" font ",14"

In this example:

• set yrange [0:1000] adjusts the range of the primary y-axis (temperature).
• set y2range [0:500000] adjusts the range of the secondary y-axis (pressure).
• set y2tics 100000, 100000, 500000 customizes the tick marks on the secondary y-axis.
• set y2label "Pressure (Pa)" font ",14" changes the font size of the y2 axis label.

Multiple Datasets on Y2 Axis

What if you need to plot multiple datasets on the secondary y2 axis? gnuplot allows you to plot multiple data sets on the same y2 axis, and it’s as simple as adding more data files to the plot command with the axes x1y2 option.

Here’s an example of plotting multiple datasets on the y2 axis:

plot "datafile1.dat" using 1:2 with lines title "Dataset 1", 
     "datafile2.dat" using 1:3 axes x1y2 with lines title "Dataset 2", 
     "datafile3.dat" using 1:4 axes x1y2 with lines title "Dataset 3"

This will plot three datasets on the graph, with the first one on the primary y-axis and the other two on the secondary y2 axis. Notice how simple it is to include multiple datasets on the secondary axis by using the axes x1y2 directive.

Conclusion

The gnuplot y2 axis is a powerful feature that allows you to display multiple datasets with different scales in a single plot. By utilizing the secondary y-axis, you can create clearer, more informative graphs that make it easier to compare and analyze data. With a little bit of practice, you can create stunning visualizations that help tell a story with your data.

Now that you know how to work with the gnuplot y2 axis, go ahead and try it out with your own datasets. Whether you’re plotting temperature and pressure, sales data and growth rates, or any other types of data, the y2 axis is a tool that can elevate your plots and make them more insightful!