atom/docs/extensions/wrap-guide.md

Wrap Guide

The wrap-guide extension places a vertical line in each editor at a certain column to guide your formatting so lines do not exceed a certain width.

By default the wrap-guide is placed at the 80th column.

Configuration

You can configure where this column is on a per-path basis using the following configuration data options:

wrapGuideConfig =
  getGuideColumn: (path, defaultColumn) ->
    if path.indexOf('.mm', path.length - 3) is -1
      return defaultColumn
    else
      return -1 # Disable the guide for Objective-C files
requireExtension 'wrap-guide', wrapGuideConfig

You can configure the color and/or width of the line by adding the following CSS to a custom stylesheet:

.wrap-guide {
  width: 10px;
  background-color: red;
}