动态画图

void CHistogramCtrl::DrawPos()
{
CRect rc;
GetClientRect(rc);
     
if (m_MemDC.GetSafeHdc() != NULL)
{

CRect rcRight = rc;
rcRight.left = rcRight.right - 1;

CBrush bkBrush(RGB(m_Back_R,m_Back_G,m_Back_B));
m_MemDC.FillRect(rcRight,&bkBrush); CPen pen_grid(PS_DOT,1,RGB(m_Mid_R,m_Mid_G,m_Mid_B));
       

m_MemDC.SelectObject(&pen_grid);

int len_x=0;
while(len_x<=rc.Height())

{
len_x+=m_nGrid;
m_MemDC.MoveTo(rcRight.left,rcRight.bottom-len_x);
m_MemDC.LineTo(rcRight.right,rcRight.bottom-len_x);

}
      CPen pen(PS_DOT,1,RGB(m_Mid_R,m_Mid_G,m_Mid_B));
       

m_MemDC.SelectObject(&pen);
m_nStep++;
if(m_nStep>m_nGrid)
{
m_nStep=0;
m_MemDC.MoveTo(rc.right-1,rcRight.top);
m_MemDC.LineTo(rc.right-1,rcRight.bottom);
} CPen pen_pos(PS_SOLID,1,RGB(m_Top_R,m_Top_G,m_Top_B));
m_MemDC.SelectObject(&pen_pos); m_MemDC.MoveTo(rc.right-1,(long) ((1-(float) (m_nOldPos - m_nLower) / m_nRange) * rc.Height()));
m_MemDC.LineTo(rc.right,(long) ((1-(float) (m_nPos - m_nLower) / m_nRange) * rc.Height()));
}
m_MemDC.BitBlt(0, 0, rc.Width(), rc.Height(), &m_MemDC, 1, 0, SRCCOPY);
}  
这是我动态画图一部分代码．为什么CPen pen_grid(PS_DOT,1,RGB(m_Mid_R,m_Mid_G,m_Mid_B));中的ＰＳ－ＤＯＴ对横格不起作用啊，画出来的是实线不是虚线．