Gnuplot 2 Graphs One Plot: A Simple Guide to Overlaying Graphs

Gnuplot is one of the most powerful tools for visualizing data and creating beautiful plots. Whether you’re working with scientific data, statistics, or simply trying to compare two sets of information, Gnuplot makes it easy. One common task that many users face is plotting multiple graphs on a single plot. This might sound like a challenge, but with the right approach, it’s quite simple! In this article, we’ll explore how to create a plot with two graphs on one plot using Gnuplot, providing step-by-step instructions and examples.

Why Overlay Two Graphs in One Plot?

When you have two datasets that are related, displaying them in a single plot can provide clear insights into their relationships. For example, you might want to compare two mathematical functions, show the trend of two data sets, or simply display a reference line alongside your data points. Overlaying two graphs can make these comparisons easier and more effective.

In Gnuplot, this is accomplished by plotting multiple datasets on the same set of axes. Let’s take a closer look at how this can be done, from the simplest examples to more advanced visualizations.

Basic Syntax for Overlaying Two Graphs

To plot two graphs on one plot in Gnuplot, you use the plot command with multiple datasets. Each dataset is specified after the plot keyword, separated by commas. Here’s the basic syntax:

plot 'datafile1.dat' using 1:2 with lines, 'datafile2.dat' using 1:2 with lines

This simple example assumes that you have two data files (datafile1.dat and datafile2.dat) and you want to plot the data from column 1 against column 2 of each file. The with lines option specifies that the data will be represented by lines. If you want to plot with points, use with points instead.

Example 1: Plotting Two Functions

Let’s start with a basic example where we want to overlay two mathematical functions on the same plot. In this case, we’ll plot the sine and cosine functions.

set title "Sine and Cosine Functions"
set xlabel "x"
set ylabel "y"
plot sin(x) with lines title 'sin(x)', cos(x) with lines title 'cos(x)'

In this example:

• set title specifies the plot title.
• set xlabel and set ylabel set the axis labels.
• sin(x) with lines and cos(x) with lines plot the sine and cosine functions, respectively.
• title 'sin(x)' and title 'cos(x)' provide a legend for each graph.

Running this in Gnuplot will produce a plot with both sine and cosine functions on the same set of axes, clearly labeled for easy comparison.

Example 2: Plotting Data from Two Files

Now let’s consider an example where we have two data files and want to plot them on the same graph. Imagine you have data in two files, data1.dat and data2.dat, each containing two columns: the x-values and the corresponding y-values.

set title "Comparison of Data Sets"
set xlabel "Time"
set ylabel "Value"
plot 'data1.dat' using 1:2 with lines title 'Dataset 1', 'data2.dat' using 1:2 with points title 'Dataset 2'

Here, using 1:2 specifies that we are using the first column for x-values and the second column for y-values. The with lines and with points specify how the data should be visualized, with Dataset 1 being represented as a line and Dataset 2 as individual points. This example also includes a title for each dataset, which will appear in the legend.

When you run this in Gnuplot, it will generate a plot with two different types of graphs: one with a continuous line and the other with individual points.

Customizing Your Plot

There are many ways to customize your plot in Gnuplot, from adjusting line colors to adding grid lines or changing the plot style. Here are some common customizations:

• Changing line styles: You can change the line style by adding linestyle options to each plot. For example, linestyle 1 might give you a solid line, and linestyle 2 a dashed line.
• Adding grid lines: The set grid command enables grid lines on your plot, making it easier to read values from the graph.
• Setting colors: You can specify colors using linecolor or lc options. For example, linecolor rgb 'red' will set the color of a line to red.
• Changing the point style: You can use the with points option and specify a point type with pointtype (e.g., pointtype 7 for a circle).

Example 3: Customizing the Graph with Colors and Grid Lines

Let’s modify our previous example to include grid lines, a customized line color, and a different point style for the second dataset:

set title "Customized Comparison of Data Sets"
set xlabel "Time"
set ylabel "Value"
set grid
plot 'data1.dat' using 1:2 with lines linestyle 1 linecolor rgb 'blue' title 'Dataset 1', 'data2.dat' using 1:2 with points pointtype 7 pointsize 1.5 linecolor rgb 'red' title 'Dataset 2'

In this case, we’ve added a set grid command to include grid lines on the plot. We’ve also specified that the first dataset will be plotted with a blue line, and the second dataset will be plotted with red points (using pointtype 7, which represents a circle).

Advanced Example: Plotting Multiple Graphs with Different Styles

Let’s look at an advanced example where we overlay several datasets with different styles. Imagine you want to plot the following:

• A sine function as a line
• A cosine function as a dashed line
• Some data from a file as points

set title "Advanced Graph Comparison"
set xlabel "x"
set ylabel "y"
set grid
plot sin(x) with lines title 'sin(x)', cos(x) with lines linestyle 2 title 'cos(x)', 'datafile.dat' using 1:2 with points pointtype 7 pointsize 1.5 title 'Data from File'

This example plots three datasets on the same graph: the sine and cosine functions as lines (with different styles) and the data from a file as points. This is a great way to overlay multiple types of data on one plot and make comparisons clear.

Conclusion

Gnuplot is an incredibly powerful tool for creating complex plots, and overlaying multiple graphs in one plot is a fantastic way to compare different datasets or functions. Whether you’re working with mathematical equations, experimental data, or simply want to visualize trends, Gnuplot makes it easy to present your data in an insightful way.

As we’ve seen, overlaying two or more graphs on one plot in Gnuplot is straightforward and can be customized in many ways. With just a few commands, you can create professional-quality plots that convey information clearly and effectively. So, go ahead and give it a try—your next graph could be the perfect comparison of two sets of data, all in one plot!