• click to add vertices
• click to (de)select a vertex and option-click another vertex to add an edge
• click to (de)select an edge and option-click an adjacent edge to fix their angle
• shift-click to place or remove the attractor, which attracts the selected vertex
• press 'f' to fix the selected vertex
• press 't' to trace the motion of the selected vertex
• press 'd' to delete the selected vertex or edge
• press 'c' to clear everything away
• press 'v' to cycle through viewing options
• press 'i' to toggle information display

press a number to load a preset linkage:

Also:

• the original python version
• github repo