There is probably a misunderstanding.
A chart that you open in Dukascopy's JForex platform (or any other broker's platform) contain prices on the selected instrument for the selected period. It always contains all markets. There is no such thing like 'chart with (without) the New York session', or 'chart with the New York closing time'. Period.
In Forex one could talk about the opening hours of the main markets. These are the London, the New York, the Sydney and the Tokyo markets' opening hours. Sometimes these opening hours are called sessions (like London session).
Probably what you have read is about WHEN to trade. (And not about on what chart to trade.) As if you trade in hours when more session are open, market volatility is higher, so it is easier to find good entry points.
More on this at several sites on internet, for example
here.