The Visualization Toolbox is a free and powerful visualization app that allows you to create highly customizable charts. It is built upon the Apache ECharts library. The Visualization Toolbox supports most of the chart types that are available in ECharts.
How to build your first EChart dashboard? Have a look at the Docs/Tutorial of the Visualization Toolbox start page in Splunk.
Code is available on Github -> https://github.com/hman-78/visualization_toolbox