Implemented the MVVM model for the chart. The underlying chart series is...

... maintained in a MarketDataGraphViewModel object. This same object contain the start and end dates.

The ListBoxItemAssetViewModel represents the objects displayed in the ListBox (list of asset codes).

The chart's theme is now maintained in a boolean wrapped in a ChartLightDarkThemeViewModel. This object is bound to the radio buttons used to select which theme to use.