The current implementation compares against the previous build of the same project when calculating Delta / Diff.
This implementation does not change that, but if you pass the new "baselinePattern" parameter to publishCppcheck in your Jenkinsfile it will calculate Diffs and changes from that baseline instead.
"baselinePattern" has the exact same semantics as the existing "pattern", i.e an xml-file generated by cppcheck. This means you can run cppcheck on almost anything during your pipeline, and compare against this.
Side note: since there were some duplicated code which needed to be run several times to implement this baseline comparison, I have also done some refactoring on this code, especially in CppcheckPublisher.