By default, ChartDirector will use the default auto scale parameters, which will start the y-axis from 0 unless the data range is small. It is because this is appropriate for most common chart applications.
However, for financial chart styles, it will not have a preference to start the y-axis from 0, because it is traditionally how financial charts are plotted.
A HLOCLayer is treated as a financial style layer, while a line layer is just a generic layer. That's why they have different auto-scale behaviour.
You may use Axis.setAutoScale to configure the auto-scale parameters to override the default. The yAxis().setAutoScale will have no effect in your code, because your code uses the secondary y-axis (yAxis2), not the default y-axis (yAxis). You may try: