void YSlider::move_to(Coord, Coord y) {
Adjustable* a = adjustable_;
a->scroll_to(Dimension_Y, a->lower(Dimension_Y) + y / yscale_);
}
void XSlider::move_to(Coord x, Coord) {
Adjustable* a = adjustable_;
a->scroll_to(Dimension_X, a->lower(Dimension_X) + x / xscale_);
}