Thank you for providing the image - I quickly reproduced the erroneous behavior on my side.
I've researched further and it seems it is caused by some XAML limitation in Xamarin.Forms as the content views inside SlideView are loaded on demand.
I can suggest the following workaround - you can define the ContentView Content as a Resource inside Page Resources or the other alternative is to define it in a separate ContentView file and reference it with <local:ChartContentView>.
I have attached a sample solution based on your code to show you the first approach, please download it and give it a try.
I hope I was of help. Let me know if you have any additional questions.
Progress is here for your business, like always. Read more about the measures we are taking to ensure business continuity and help fight the COVID-19 pandemic. Our thoughts here at Progress are with those affected by the outbreak.