To use pikchr you really need to downlolad it, play with the examples and read the documentation, but an example pikchr source file (input.pikchr) such as shown below is a great exemplar of the Diagrams as Code paradigm.
box color blue "A" fit;
circle color red "C"
arrow <-> "double headed" "arrow" width 200% chop;
circle "Small" fit;
box "long text will expand the box" fit;
By the use of the command pikchr --svg-only input.pikchr > image.svg gives the image in svg format, ideal for just pasting into a markdown document to display on a web page.