After spending too many hours trying to figure this one out, I have to write it down to help any unfortunate soul that should come across this undocumented behavior. (Notice how I put all the important keywords in the title?)
While it's most often recommended to supply an IList<T> when setting the ItemsSource of a collection view in Xamarin.Forms or other XAML platforms, it's not required. You can just as well supply an or IEnumerable<T> and things will work fine.
The benefit if passing a list is that the system can do random access of items. In other words, it can use the mylist[42] syntax. This is a lot more efficient than trying to access a random item in an IEnumerable<T>.
You might think "but there's ElementAt() on IEnumerable<T>". Yes, but that's basically just skipping n - 1 items before accessing the desired item. myEnumerable.ElementAt(42) is basically this: myEnumerable.Skip(42 - 1).First().
In the Xamarin.Forms documentation it actually explictly tells you to provide a list if you can, for performance reasons.
Back to SfChart by Syncfusion:
Apparently there's a limitation in SfChart when it comes to IEnumerable<T> as the ItemsSource: If you specify a custom TrackballLabelTemplate for your chart, it will be ignored if you don't use an IList<T>.
This had me stuck for quite a while.
I usually provide a list when binding to ItemsSource, but in this instance I was using Where() in the view code-behind to filter out some unwanted items. Since Where() returns an IEnumerable<T> things didn't work. Putting a ToList() after the Where() clause fixed it all.
This wasted more hours than I want to admit to figure out...
If you stumble upon this post via Google; you're welcome :)
Happy coding!
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