How to Add a permanent contour line to a surface plot in R plotly

Last Updated : 24 Apr, 2025

A surface plot is a 3D plot that displays a surface in a three-dimensional space. It is a useful visualization technique for representing mathematical functions or data with two continuous variables. A contour line, on the other hand, is a line that connects points of equal value in a two-dimensional plot. Adding a contour line to a surface plot can enhance its visualization by highlighting the areas of equal value on the surface.

The plotly library in R provides an easy way to create surface plots with customizable contour lines. In this article, we will explain the theory behind adding a permanent contour line to a surface plot using the plotly library, and provide 3-4 examples of R code with detailed explanations.

A surface plot in R Plotly is a three-dimensional plot that represents a function of two input variables (usually, x and y) and one output variable (usually, z). The plot shows the surface of this function as a mesh of triangles, with the height of each triangle representing the value of the output variable.

A contour line, on the other hand, is a curve that connects points of equal value on the surface plot. These lines are useful for visualizing the shape and structure of the function, as well as for identifying areas of interest or anomalies in the data.

To add a permanent contour line to a surface plot in R Plotly, we can use the contours argument in the plot_ly() function. This argument allows us to specify the contour levels we want to show, as well as various properties of the contour lines such as their width and color.

By setting the contours argument to list(z = list(show = TRUE, width = 2, coloring = "lines")), we can add contour lines to the plot that connect points of equal value on the surface. We can adjust the appearance of the contour lines by specifying their width and coloring properties. For example, we can set width = 2 to make the lines thicker, or coloring = "heatmap" to color them based on the heatmap color scale of the surface plot.

In addition, we can also set the highlightcolor property to change the color of the contour line at the value of interest. This can be useful for highlighting specific features or anomalies in the data.

Overall, adding a permanent contour line to a surface plot in R Plotly is a simple yet powerful way to enhance the visualization and gain deeper insights into the underlying data.

To add a permanent contour line to a surface plot using plotly, we need to use the add_trace() function to add a contour trace to the plot. The add_trace() function allows us to add additional traces to a plot created using the plot_ly() function. A trace is a set of data and visual properties that are used to create a plot.

The add_trace() function takes several parameters, including the data and visual properties of the trace. In the context of adding a permanent contour line to a surface plot, we need to set the type parameter to "contour" and provide the z values for the contour line. We can also customize the appearance of the contour line using the contours parameter.

Example 1: Basic Surface Plot with Contour Line
In this example, we will create a basic surface plot with a permanent contour line at a fixed height using the plotly library in R. Here is the code:

library(plotly)  # Create a surface plot p <- plot_ly(z = volcano, type = "surface")  # Add a contour trace with a fixed z value p <- add_trace(p, z = matrix(120, nrow = nrow(volcano), ncol = ncol(volcano)), contours = list(coloring = "none"))  # Show the plot p

Output:

In this code, we first load the plotly library using the library() function. We then create a surface plot of the volcano dataset using the plot_ly() function. The z parameter is set to volcano, which is a matrix of altitude values. The type parameter is set to "surface" to create a surface plot.

We then add a contour trace to the plot using the add_trace() function. The z parameter is set to a matrix with a fixed value of 120, which represents the height of the contour line. The contours parameter is used to customize the contour line by setting coloring to "none", which means that the contour line will have a single color.

Finally, we show the plot using the p object.

Example 2: Surface Plot with Customized Contour Line

In this example, we will create a surface plot with a customized contour line using the plotly library in R. Here is the code:

library(plotly)  # Create a surface plot with a dynamic contour line p <- plot_ly(z = volcano, type = "surface", contours = list(z = list(show = TRUE)))  # Show the plot p

Output:

We then add a dynamic contour line to the plot using the contours parameter in plot_ly(). The z parameter in contours is set to list(show = TRUE) to show the contour lines. This will create a default number of contour lines based on the range of the z values.

Finally, we show the plot using the p object.

Note that we did not use the add_trace() function in this code because we added the surface trace and the contour lines together using the contours parameter in plot_ly(). This is what makes the contour lines dynamic because they are based on the z values of the surface plot.

Adding features to make more attractive:

library(plotly)  # Create a surface plot with a dynamic contour line p <- plot_ly(z = volcano, type = "surface", colors = "Blues",               colorbar = list(title = "Elevation", len = 0.5),               contours = list(z = list(show = TRUE, width = 2,                                        highlightcolor = "#ff0000",                                         project = list(z = TRUE)),                               coloring = "heatmap"))  # Set the plot layout p <- layout(p, title = "Volcano Surface Plot with Dynamic Contour Lines",             scene = list(xaxis = list(title = "X-axis"),                          yaxis = list(title = "Y-axis"),                          zaxis = list(title = "Elevation"),                          aspectratio = list(x = 1, y = 1, z = 0.5)))  # Show the plot p

Output:

In this code, we have added the following features to the surface plot with dynamic contour lines:

colors: The colors parameter is set to "Blues" to change the color scheme of the surface plot to shades of blue.

colorbar: The colorbar parameter is used to add a color bar to the plot with a title and a custom length of 0.5.

width: The width parameter in contours is set to 2 to increase the width of the contour lines.

highlightcolor: The highlightcolor parameter in contours is set to "#ff0000" to highlight the contour lines in red.

project: The project parameter in contours is set to list(z = TRUE) to project the contour lines onto the z-axis.

coloring: The coloring parameter in contours is set to "heatmap" to color the contour lines based on the color scale of the surface plot.

We have also set the layout of the plot using the layout() function. The title parameter is used to add a title to the plot, and the scene parameter is used to customize the x-axis, y-axis, and z-axis of the plot. The aspectratio parameter is used to set the aspect ratio of the plot to 1:1:0.5.

Finally, we show the plot using the p object.

Conclusion:

adding a permanent contour line to a surface plot in R Plotly can greatly enhance the visualization of the underlying data and help identify important features and anomalies. This can be achieved by using the contours argument in the plot_ly() function and specifying the contour levels, width, coloring, and highlight color properties. With this simple technique, we can create a more informative and visually appealing plot that can be used for a variety of applications, from scientific research to data exploration and communication.

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How to Add a permanent contour line to a surface plot in R plotly

Example 2: Surface Plot with Customized Contour Line

Adding features to make more attractive:

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