hi again^^,
I think i found a possible bug:
For the purpose of reach the illustreted result, i override the timeAxis class in particular:

protected override string FormatValueOverride (double x)

in which i put in the string:

fmt = fmt.Replace("M",""+ CultureInfo.CurrentCulture.DateTimeFormat.GetAbbreviatedMonthName (time.Month).ToUpper()+"");

It work but it changes some letter of the month name in others letter/numbers like:
OcA <- (should be Oct)   5a14 <- (Mar) 3eb <- Feb   etc..
Other are ok like Sep Jan Jun Jul etc

Someone can suggest me a solution? 
Thank you

thank you for the quick aswer!
I'm using the Xamarin.Ios Nuget Package.
This morning i have updated xamarin at the last version and this solved the circle problem!
Now i can see the circle markers and the rect around the chart appeared too, so i used PlotAreaBorderThickness = new OxyThickness(0) getting rid of it! 
So guys if someone has the same problem, updating xamarine.ios solve that!




Thank you very much for the help!!