onmousedown="setPosition(evt, true)"
     onmouseup="setPosition(evt, false)"
     onmousemove="moveViewPort(evt)"
     onload="init(evt)">

var track_mouse_position = false;
var original_mouse_position = new Array();
var SVGRoot;

function init(evt) {
  SVGRoot = evt.target.ownerDocument.documentElement;
}

function setPosition(evt, active) {
  track_mouse_position = active;
  if (track_mouse_position) {
    original_mouse_position['x'] = evt.clientX;
    original_mouse_position['y'] = evt.clientY;
    SVGRoot.cursor = 'move';
  } else {
    original_mouse_position = new Array();
    SVGRoot.cursor = '';
  }
}

function moveViewPort(evt) {
  if (track_mouse_position) {
    viewBox = SVGRoot.getAttribute('viewBox');
    var viewVals = viewBox.split(' ');
    viewVals[0] = original_mouse_position['x'] - evt.clientX;
    viewVals[1] = original_mouse_position['y'] - evt.clientY;
    SVGRoot.setAttribute('viewBox', viewVals.join(' '));
  }
}