title: Add horizontal line to chart in C#
body: <p>I am using a <code>System.Windows.Forms.DataVisualization.Chart</code> to plot some x,y scatter data, like this:</p>
  
  <pre><code>chart1.Series["Series2"].Points.AddXY(stringX, doubleY);
  </code></pre>
  
  <ol>
  <li><p>I would like to add to that chart an horizontal line with an average of the form y = constant. <strong>How can I do this? Please, notice that the x axis is a string</strong></p></li>
  <li><p>Actually, the x axis is the time (hh:mm:ss). I am converting it to string to plot it because if I use the DateTime format for the x axis (XValueType) of the chart it doesn't show the seconds. <strong>Can I correct that to show the seconds so I can plot directly x as DateTime and y as double?</strong></p></li>
  </ol>