The space you mentioned in due to the y-axis margin, which shorten the scale of the y-axis a little bit. So there is an empty region with no scale at the top. If you set the y-axis margin to 0, the gap will disappear.
In the default implementation of the FinanceChart, we put the legend box inside the plot area at the top. This can cause the legend box to overlap with the chart contents. So we also set an axis margin to leave some empty space at the top.