gtk:level-bar-add-offset-value (levelbar name value)

levelbar -- a gtk:level-bar widget
name -- a string for the name of the new offset
value -- a double float for the new offset

Adds a new offset marker on the level bar at the position specified by value. When the level bar value is in the interval topped by value, or between the value and max-value values in case the offset is the last one on the bar, a style class named level-name will be applied when rendering the level bar fill. If another offset marker named name exists, its value will be replaced by value.