That's a good question. The dotted line shows the close price of the day before. For certain products that are open almost 24h a day, it's the price at 0.00h in the night (where the new day starts).
Basically, the chart visually shows the daily difference that you can also see in percent in the market screen.
If you have any further questions in regards to this, please feel free to contact us via email at BUX Support.