KeenWrite is my free, open-source, cross-platform desktop editor for R Markdown and Markdown documents. The latest release provides users with functionality to embed R plots into documents. I started developing the editor because no other text editors make it trivial to define and use variables within a document. In KeenWrite, inserting a variable is accomplished by typing in a partial value followed by pressing Control+Space.
Typesetting is accomplished using ConTeXt, which must be installed separately.