\horizontal_space is a page layout operator. It is used only inside the diagram header block. It defines the minimal horizontal space left between two steps in a same line. It is followed by an integer representing the height of this blank space in millimeter. Considering the width of steps, the system can increase this value to make diagram presentation more pleasant.
\horizontal_space(5); % left a space of 5 mm between two consecutive steps
\horizontal_space(WIDTH);
WIDTH Description : Indicates the space between two consecutive steps. The value is given in millimeter. Type : Integer. Allowed values : Any positive integer value in range [0, 210].