You can achieve this by using StripLine
.
// Create the stripline for 'gold status'
StripLine sline = new StripLine();
sline.IntervalOffset = 0.6;
sline.StripWidth = 0.25;
sline.Text = "Gold Status";
sline.Interval = 0.0; // this ensures it will not repeat like normal strip lines
sline.BackColor = Color.Yellow;
myChart.AxisY.StripLines.Add(sline);
repeat the process for your silver and platinum strip lines.