You have hit on quite a contentious area I’m afraid. The simple answer is, yes it is normal.
To elaborate, there is a pervasive school of thought that says if you add connecting lines through months where:
- no data has been added intentionally
- no data has been added unintentionally
- a NULL result has been added
- a N/A result has been added
then any straight line misrepresents the actual situation.
The second, school of thought says, but the connecting line will not have any data marks (circles).
The software design has taken the absolute route that is not open to interpretation, therefore no connecting line. I agree it does look a bit odd, but it does represent the data exactly.