Изменение значений переменных - C (СИ)

void drawgraphic(double min_x, double max_x)
{
    double min_y = 1000000.0, max_y = -1000000.0;
    double x = 0, dx = (max_x - min_x)/640.0;

    for (x = min_x; x <= max_x; x+=dx)
    {
        double y = func(x);
 
        if (y < min_y)
            min_y = y;
        else if (y > max_y)
            max_y = y;
    }
 
    //Viewport setting
    _setwindow(1, min_x, max_y, max_x, min_y);
 
    _setcolor(4);
    _moveto_w(min_x, func(min_x));
    for (x = min_x; x < max_x; x+=dx)
    {
        _lineto_w(x, func(x));
    }
 
    _setcolor(1);
    _moveto_w(min_x, 0); _lineto_w(max_x, 0);
    _moveto_w(0, min_y); _lineto_w(0, max_y);
 
}