A control similar to SGI's Inventor toolkit, commonly used to move 3D objects or control zoom.
You can use set_vertical() to make the thumbwheel move vertically.
Fltk calls this virtual function to draw the widget, after setting up the graphics (current window, xy translation, etc) so that any drawing functions will go into this widget.
User code should not call this! You probably want to call redraw().
The default version calls draw_box() and draw_label(), thus drawing the box() to fill the widget and putting the label() and image() inside it to fill it, unless the align() flags are set to put it outside.
Information on how to write your own version is here.
Reimplemented from fltk::Widget.
The base class handle() accepts FOCUS and recognizes a number of keystrokes that adjust the value. A subclass can call this to get these keystrokes, it can also do it's own keystroke handling if it wants.
Reimplemented from fltk::Valuator.