The 5.5 branch introduced several refinements that have since matured into the stable 6.0 release:
The refers to the development version of the legendary command-line driven graphing utility. While gnuplot 5.4 was the long-standing stable branch and gnuplot 6.0 is the current primary stable release (as of December 2023), version 5.5 served as the bridge where many cutting-edge features—such as data-driven histogram coloring and enhanced coordinate handling—were first tested. How to Download Gnuplot 5.5 gnuplot 5.5 download
Occasionally, development "snapshots" are contributed to the Gnuplot SourceForge Files page under the "development" or "testing" folders. gnuplot 5.5 download