Figures generated by R code not rendered properly with Travis/ GitHub Pages

We're using Bookdown to edit and manage a version of a textbook hosted on GitHub Pages and deployed by Travis. It appears that our Travis build process is not properly rendering figures generated by R code (well, some figures; we're not sure what's going on).

When we serve the book locally, we see the correct figure. When we view it on GitHub pages the image is there, but it's wrong. Here is our repo: https://github.com/nulib/moderndive_book

Figure 2.22 is blank on the online version. It's a .png file of a plot, but the plot is empty. The image should look like Figure 2.22 here: https://moderndive.com/2-viz.html#boxplots

I fear that it might be an issue with our .gitignore file, or our .travis configuration. Any advice here?