I found the solution to the problem after much tinkering and googling. What I ended up doing was specifying 2 points on either side of the graph and then drawing a line between them. This is done in the 'Post Paint' event of the chart found in the Advanced section.
I also added a parameter in the parameters section which contains the Y-Axis constant for the trend line.
See the below code for the solution:
// Parameter: chartObj - represents the chart object
// Parameter: sender - the chart object that will be painted
// Parameter: e - arguments that contain the graphics object
// Parameter: codeParams - user defined code parameters
double stripValue = Double.Parse(codeParams["Std"].ToString());
PointF p1 = new PointF();
p1.X = (float)chartObj.ChartAreas["Default"].AxisX.Minimum;
p1.X = (float)chartObj.ChartAreas["Default"].AxisX.ValueToPixelPosition(p1.X);
p1.Y = (float)chartObj.ChartAreas["Default"].AxisY.ValueToPixelPosition(stripValue);
PointF p2 = new PointF();
p2.X = (float)chartObj.ChartAreas["Default"].AxisX.Maximum;
p2.X = (float)chartObj.ChartAreas["Default"].AxisX.ValueToPixelPosition(p2.X);
p2.Y = (float)chartObj.ChartAreas["Default"].AxisY.ValueToPixelPosition(stripValue);
e.ChartGraphics.Graphics.DrawLine(new Pen(Color.FromArgb(255, 0, 0, 0), 1), p1, p2);