private void timer_Tick(object sender, EventArgs e)
    {
        timer.Stop();
        for (int i = 0; i < TOTAL_SENSORS; i++)
        {
            DateTime d = DateTime.Now;
            devices[i].Value = float.Parse(serialPort.ReadLine());
            if (chart1.Series[i].Points.Count > MAX_POINTS)
            {
                //see the most recent points
            }
            chart1.Series[i].Points.AddXY(d, devices[i].Value);
        }
        timer.Start();
    }

Dictionary<DateTime, float> points = new Dictionary<DateTime, float>();

points.Add(d, devices[i].Value);

points.Remove(points.Keys[0]);

IEnumerable<KeyValuePair<DateTime, float>> mostRecent = points.Skip(points.Count - 10).Take(10);

float value = points[DateTime.Now.AddMinutes(-1)];

foreach(KeyValuePair<DateTime, float> point in points)
{
    DateTime time = point.Key;
    float value = point.Value;
}