UI.HSlider

static bool HSlider(string id, Single& value, float min, float max, float step, float width)

   
string id A per-window unique id for tracking element state.
Single& value The value that the slider will store slider state in.
float min The minimum value the slider can set, left side of the slider.
float max The maximum value the slider can set, right side of the slider.
float step Locks the value to intervals of step. Starts at min, and increments by step.
float width Physical width of the slider on the window.
RETURNS: bool Returns true any time the value changes.

A horizontal slider element! You can stick your finger in it, and slide the value up and down.