gtk:scale-add-mark (scale value pos &optional markup)

scale -- a gtk:scale widget
value -- a number coerced to a double float with the value at which the mark is placed, must be between the lower and upper limits of the adjustment of the scale
pos -- a value of the gtk:position-type enumeration
markup -- a string with the text to be shown at the mark, using Pango markup, or the default nil value

Adds a mark at value. A mark is indicated visually by drawing a tick mark next to the scale, and GTK makes it easy for the user to position the scale exactly at the marks value. For a horizontal scale, :top and :left are drawn above the scale, anything else below. For a vertical scale, :left and :top are drawn to the left of the scale, anything else to the right.

If the markup argument is not nil, text is shown next to the tick mark. To remove marks from a scale, use the gtk:scale-clear-marks function.