That is because you have not set a Tic frequency value for your slider control. Means you must set a value to the slider control saying that your one Tic may move the control to just one position. For that write the code as below
m_slidervertical.SetBuddy(&m_ChartCtrl, FALSE);
m_slidervertical.SetRange(-10, 10);
slidervertical.SetTicFreq( 1 );
m_slidervertical.SetPos(0);