For more details see Legends
for (int i = 0; i < 1; i++)
{
chart1.Series[0].Points.AddXY("Jan", i + 10);
chart1.Series[0].Points.AddXY("Feb", i + 15);
}
// Create a new legend called "Legend1".
chart1.Legends.Add(new Legend("Legend1"));
// Set title
chart1.Legends["Legend1"].Title = "My legend";
// Assign the legend to Series1.
chart1.Series["Series1"].Legend = "Legend1";
chart1.Series["Series1"].IsVisibleInLegend = true;