val(name + "_eqs"); ++i) {
        context.beginPath();
        var eq = eval(name + "_eq_" + (i+1));
        var xys = get_points_on_the_ends(eq[0], eq[1], eq[2]);
        context.moveTo(x_to_client(xys[0][0]), y_to_client(xys[0][1]));
        context.lineTo(x_to_client(xys[1][0]), y_to_client(xys[1][1]));
        context.strokeStyle = line_colors[i];
        context.lineWidth = 2;
        context.stroke();
        context.closePath();
        context.lineWidth = 1;
    }
    
    // grid
    draw_grid_on(