\vertical_space is a page layout operator. It is used only inside the diagram header block. It defines the minimal vertical space left between two lines of steps. It is followed by an integer representing the width of this blank space in millimeter. Considering the height of steps, the system can increase this value to make diagram presentation more pleasant.
\vertical_space(15); % left a space of 15 mm between two steps lines
\vertical_space(HEIGHT);
HEIGHT
Description : Indicates the space between two consecutive steps lines.
The value is given in millimeter.
Type : Integer.
Allowed values : Any positive integer value in range [0, 297].