If you define the chart in a new window and open it from the app main window, and then close the window, the chart stays in memory. This is reproducible only in a data binding scenario - the ItemsSource of the chart series should be set to a ObservableCollection<T>.
Additionally, the collection bound to the ItemsSource of the series should be still alive. For example, it can be defined in the view model of the main window that opens the window with the chart.
To resolve this you will need to set the ItemsSource of all chart series in the window to null.
The fix for this issue will be available with LIB (version 2019.1.204) scheduled for publishing on Monday, 4th February 2019.