Of course, you can again inherit from a subclass, making this class the superclass of the new subclass. This leads to a hierarchy of superclass/subclass relationships.
If you draw this hierarchy you get an inheritance graph.
In the literature you also find illustrations where the arrowed lines are used just the other way around. The direction in which the arrowed line is used, depends on
how the corresponding author has decided to understand it.
In this tutorial the arrowed line is always directed towards the superclass.
In the further discussion an unmarked arrowed line indicates “inherit-from''.