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.