in class TChart, 
there is such code:

    protected void initVars()
    {
...
NSNotificationCenter.DefaultCenter.AddObserver((NSString)"UIDeviceOrientationDidChangedNotification",
            delegate {
            this.chart.Invalidate();
         });
...


that code has two problems:
1. "UIDeviceOrientationDidChangedNotification" should be "UIDeviceOrientationDidChangeNotification"
2. added observer, when init, however, haven't unregister it when dispose this object. thus DefaultCenter will always refer to the delegate method, which caused gc will not collect the Tchart object. I think it should be like this:

private readonly NSObject notification;

    protected void initVars()
    {
...
notification = NSNotificationCenter.DefaultCenter.AddObserver((NSString)"UIDeviceOrientationDidChangedNotification",
            delegate {
            this.chart.Invalidate();
         });
...
  }

        protected override void Dispose(bool disposing)
        {
            if (disposing)
            {
                notification.Dispose();
            }

            base.Dispose(disposing);
        }



ref:
https://developer.apple.com/documentation/uikit/uideviceorientationdidchangenotification