Question

I may be asking the wrong question, but what I need is to add a “Guide Line” to my windows form chart. In other words I have a chart with a simple data series and I need to draw a line on the y axis at the passing score, or 80%. I don’t want to add a second series as the first series has an undetermined number of data points. Is there a simple way to simply draw one line on the y axis?
The dashed line below is what I am shooting for(it does not need the arrows).

100|
   |
 90|
   |                     o
 80|<----------------------->
   |
 70|      o                  o
   |
 60|         o
   |   o        o
 50|o              o
   |_________________________
    1  2  3  4  5  6  7  8  9

Answer 1

You can add a StripLine.

Use StripWidth property to set line position:

var series = chart1.Series[0]; //series object
var chartArea = chart1.ChartAreas[series.ChartArea];
chartArea.AxisY.StripLines.Add(new StripLine
                                           {
                                               BorderDashStyle = ChartDashStyle.Dash,
                                               BorderColor = Color.DarkBlue,
                                               StripWidth = 80//Here is your y value
                                           });

UPDATE: Previous version of this answer used Interval instead of StripWidth. As @dthor correctly pointed out in the comments setting the Interval will draw a repeated strip line. In the example above, Interval is set to 0 by default.