How do I configure asciidoctor-gradle-plugin to handle plantuml?

I don't know any Ruby but I do like asciidoctor (and the gradle plugin). Is there a simple way to get the plugin to generate plantuml diagrams?

I looked at the basic asciidoctor-diagrams functionality which seems specific to using the native/ruby asciidoctor extension (which I don't have and would rather not install since I like using the gradle plugin).

The nicely done asciidoctor-gradle-plugin documentation page shows that it accepts a requires option for Ruby modules but having no grounding in Ruby whatsoever, I'm not sure if that's what I need or not.

I tried the path of least resistant, i.e.:

asciidoctor {    logDocuments = true    separateOutputDirs = false    sourceDir = file("src")    outputDir = file("$buildDir/html")    backends = [ 'html5' ]    requires "asciidoctor-diagram"}

Against my basic plantuml test:

.The PlantUML block extension class[plantuml, sample-plantuml-diagram, alt="Class diagram", width=135, height=118]----class BlockProcessorclass PlantUmlBlockBlockProcessor <|-- PlantUmlBlock----

But got:

* What went wrong:Execution failed for task ':asciidoctor'.> (LoadError) no such file to load -- asciidoctor-diagram

during the configuration phase.

How can I configure asciidoctor-gradle-plugin to handle plantuml?