Be careful about content of these custom files because deploy of these files can cause security vulnerability of Graphlytic.
Prepare your widget files. Supported file extensions are: css, html, js
Choose widget ID (e.g. myWidget). Do not use "_" as prefix.
Name your files using your widget ID, e.g. "myWidget.css", "myWidget.html" and "myWidget.js". You can have only 1 file of each type (css, html, js) but you don't need to have all types, so your widget can be only the js file when you don't need custom html and css functionality.
There is no need to stop Graphlytic.
Locate "widgets" directory, see "graphlytic.conf" to find where is the location of this directory.
Create subdirectory in this directory using your widget ID as the subdirectory name, e.g. "myWidget".
Copy prepared files into this created subdirectory.
Enable and configure widget in Widgets settings.