Behind the scenes, the Plot function is what is responsible for executing and plotting each function:

private void Plot(Func<double, double> fn, string fnName, 
  Color graphColor, GraphPane gpane = null)
{
  const double xmin = -2.0;
  const double xmax = 2.0;
  const int resolution = 2000;
  zed.IsShowPointValues = true;
  zed.PointValueFormat = "e";
  var pane = gpane ?? zed.GraphPane;
  pane.XAxis.MajorGrid.IsVisible = true;
  pane.YAxis.MajorGrid.IsVisible = true;
  pane.Title.Text = fnName;
  pane.YAxis.Title.Text = string.Empty;
  pane.XAxis.Title.Text = string.Empty;
  double[] xarr = new double[resolution];
  double[] yarr = new double[resolution];
  double incr = (xmax - xmin) / resolution;
  double x = xmin;
for(int i=0; i < resolution; i++, x+=incr)
{
  xarr[i] = x;
  yarr[i] = fn(x);
}
PointPairList list1 = new PointPairList(xarr, yarr);
LineItem li = pane.AddCurve(string.Empty, list1, graphColor,
  SymbolType.None);
li.Symbol.Fill = new Fill(Color.White);
pane.Chart.Fill = new Fill(Color.White, 
  Color.LightGoldenrodYellow, 45.0F);
}

The main point of interest within this code is highlighted in yellow. This is where the activation function that we passed in is executed and its value used for the y axis plot value. The famous ZedGraph open source plotting package is used for all graph plotting. Once each function is executed, the respective plot will be made.