for (double j = xmin; j < xmax; j += 1)
            {

                double yy = -equationSolve(j);

                chart1.Series[0].Points.AddXY(j, yy);

            }

       double equationSolve(double x)
        {
            double answ = 0;

            for (int i = 0; i <= degree; i++)
            {
                double c = coeffs[i];
                if (degree != i)
                    c *= Math.Pow(x, degree - i);

                answ += c;
            }
            return answ;
        }

var koef = textBox2.Text.Split(',');
foreach (var inf in koef) coeffs.Add(Convert.ToDouble(inf.Replace(".", ",")));

После чего строю график :
double equationSolve(double x)
{
double answ = 0;

for (int i = 0; i <= degree; i++)
{
double c = coeffs[i];
if (degree != i)
c *= Math.Pow(x, degree - i);

answ += c;
}
return answ;
}

Plot(){
chart1.Series[0].Points.Clear();
chart1.Series[0].ChartType = System.Windows.Forms.DataVisualization.Charting.SeriesChartType.Spline;
chart1.ChartAreas[0].AxisX.Crossing = 0;
chart1.ChartAreas[0].AxisY.Crossing = 0;
chart1.ChartAreas[0].AxisX.IntervalOffset = 0;
chart1.ChartAreas[0].AxisY.IntervalOffset = 0;
chart1.ChartAreas[0].AxisY.Interval = 1;
chart1.ChartAreas[0].AxisX.Interval = 1;
chart1.ChartAreas[0].AxisX.LineWidth = 3;
chart1.ChartAreas[0].AxisY.LineWidth = 3;
// chart1.ChartAreas[0].AxisX.MajorGrid.LineDashStyle = "";


var xmax = Convert.ToInt32(textBox1.Text);
var xmin = Convert.ToInt32(textBox3.Text);

chart1.ChartAreas[0].AxisX.Maximum = xmax;
chart1.ChartAreas[0].AxisX.Minimum = xmin;




xmax++;

for (double j = xmin; j < xmax; j += 1)
{

double yy = equationSolve(j);

chart1.Series[0].Points.AddXY(j, yy);

}

}