Creating beautiful charts with JRuby and JFreeChart

Summary

I recently returned from RubyKaigi where I had the opportunity to sit down with members of the Japanese Ruby community and show them a little bit of JRuby. One of the items that came up a few times was the difficulty of utilizing external libraries from Ruby: if it’s a C library, typically you have to either write a C extension or do the extra work of writing up an FFI binding. If the library is not implemented in C or Ruby, things get even weirder. One example is the Charty library, one of the more popular options for generating beautiful chart graphics on Ruby. But Charty actually wraps a Python library called matplotlib, and the bindings for CRuby literally load Python into the current process and call it via Ruby C API calls which then make Python C API calls. The horror! Also recently, a Reddit Ruby post demonstrated the use of the QuickChart library, which is implemented in JavaScript… yuck! In this case, the call-out is via process launching to the QuickChart command line, but either way it’s not at really integrated into the Ruby app using it. Is there a better way? You bet there is: use a JVM-based library from JRuby! This post will show you how to get started with JRuby’s Java integration and a few quick examples of using the JFreeChart library from Ruby code. I promise you won’t have to write a single line of Java, C, Python, or JavaScript. JFreeChart The JFreeChart library provides an extensive set of chart formats, with a huge array of rendering options and easy integration with either JVM-based GUI toolkits or simple image-file output. Unlike many of the other charting libraries, it also supports live updating of a given chart GUI, making it an excellent choice for desktop monitoring tools. Building a GUI with JRuby is fun and easy, but for this post we’ll focus on just the chart and image generation parts of the API and how you can use them easily from Ruby. JRuby’s Java Integration The magic starts with JRuby’s Java integration layer. The simplest wa...