I am using 2.6.2 and our requirement is to display asterisk in place of missing bars. I tried replacing null values with asterisk in afterDatasetFilled function, but chart does not display non-numerical characters.
Replacing the null value with zero and suffixing it with asterisk in the chart option produced a bar with height 1, which was not acceptable.
Attached is a screenshot of how the chart needs to look based on our requirement.
If you can replace the null value with a value that will not show up in
your dataset then you could use a bit of script to hide the bar and
replace the label with an *. Take a look at the attached example.