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nullFormat and nullFormater don't work in this case, because they aren't available for this type of series. I know you can find them in API, but that is most likely a docs bug and it will be reported.
These options work properly for series like heatmap and tilemap, where you can easily tell where null value is located. For series like line it would be hard, because null values are simply represented by breaks in the line.
Let me know if you have any further questions!
Best regards!
It could be possible but could you explain how do you see it? Where should be that tooltip located? How the path should be connected? Any kind of illustration would be greatly appreciated here, because I cant really understand where would you locate those elements. As I have mentioned before, in this type of series null values are shown by breaks in the line (https://jsfiddle.net/BlackLabel/x6u539cy/).
Thanks in advance for any additional informations.
In the above fiddle when using connectNulls true option and hovering those values tooltip is not showing up.
For example, When hover on Jan 3rd/Jan 4th tooltip is showing for Jan2nd value. Is there any possibility to show tooltip for Jan 3rd and 4th where the value is null?