When a Ruby ecosystem library is missing or weak, JRuby lets you pull any JVM library (Maven Central + other repos — probably the largest library collection on any language platform) via a `jar-file` declaration and `lock_jars` (analogous to Gemfile/bundle install). Imported classes behave like Ruby classes with case-conversion; once imported you may forget they're Java. Example: JFreeChart generates 3D PNG/SVG/PDF charts in ~10 lines of Ruby.