ui/src/main/java/ucar/nc2/ui/geoloc/Navigation.java
public void pan( double deltax, double deltay) {
    zoom.push();

    pix_x0 -= deltax;
    pix_y0 -= deltay;
    fireMapAreaEvent();
  }
public void pan( double deltax, double deltay) {
    zoom.push();

    pix_x0 -= deltax;
    pix_y0 -= deltay;
    fireMapAreaEvent();
  }
call this to change the center of the screen's world coordinates.
deltax, deltay in display coordinates