init_runge(){
    runge = document.getElementById("runge");
    var canvas_rect = runge.getBoundingClientRect();
    runge_draw(0, 0, false);

    runge.addEventListener('mousemove', function(e){
        var canvas_rect = runge.getBoundingClientRect();
           runge_draw(e.clientX - canvas_rect.left, e.clientY - canvas_rect.top, true);
           if(e.buttons == 1){
            runge_xs[runge_i] = client_to_x(e.clientX - canvas_rect.left);
            runge_ys[runge_i] = client_to_y(e.clientY - canvas_re