https://stackoverflow.com/questions/12422398/drawing-line-on-a-click-on-zedgraph-pane
public Form1()
{
InitializeComponent();
}
PointPairList userClickrList = new PointPairList();
LineItem userClickCurve = new LineItem("userClickCurve");
private void zedGraphControl1_MouseClick(object sender, MouseEventArgs e)
{
// Create an instance of Graph Pane
GraphPane myPane = zedGraphControl1.GraphPane;
// x & y variables to store the axis values
double xVal;
double yVal;
// Clear the previous values if any
userClickrList.Clear();
myPane.Legend.IsVisible = false;
// Use the current mouse locations to get the corresponding
// X & Y CO-Ordinates
myPane.ReverseTransform(e.Location, out xVal, out yVal);
// Create a list using the above x & y values
userClickrList.Add(xVal, myPane.YAxis.Scale.Max);
userClickrList.Add(xVal, myPane.YAxis.Scale.Min);
// Add a curve
userClickCurve = myPane.AddCurve(" ", userClickrList, Color.Red, SymbolType.None);
zedGraphControl1.Refresh();
}
you just have to change the userClickList to draw horizontal line.