Bump org.jfree:jfreechart from 1.5.5 to 1.5.6 (#191)

Bumps [org.jfree:jfreechart](https://github.com/jfree/jfreechart) from 1.5.5 to 1.5.6.
- [Release notes](https://github.com/jfree/jfreechart/releases)
- [Commits](jfree/jfreechart@v1.5.5...v1.5.6)

---
updated-dependencies:
- dependency-name: org.jfree:jfreechart
  dependency-version: 1.5.6
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Author: dependabot[bot]

plugin/pom.xml

@@ -128,7 +128,7 @@
         <dependency>
             <groupId>org.jfree</groupId>
             <artifactId>jfreechart</artifactId>
-            <version>1.5.5</version>
+            <version>1.5.6</version>
         </dependency>
 
         <dependency>