Vector of buttons.  Used by CENSUSYQ3W5V demo.

  BUTTONV([xmin ymin],k1:k2,ks,'callback', ... )

  The lower left corner is [xmin ymin] in figure normalized coordinates.

  The buttons are labeled with integers k1:k2.

  Initially, button ks is highlighted.

  After each button push, a variable named button is set to the

  button index and the string 'callback' is evaluated.

  Up to 3 additional optional arguments are extra button labels.