"@signal" and "@axes" not defined in NXdata group "/scan/data"

[jkotan]

I get

        NXdata: /scan/data
            "@signal" not defined in NXdata group "/scan/data"
            "@axes" not defined in NXdata group "/scan/data"

However, on https://manual.nexusformat.org/classes/base_classes/NXdata.html @signal and @axes are mentioned as optional .i.e.

DATA: (optional) [NX_NUMBER](https://manual.nexusformat.org/nxdl-types.html#nx-number) (Rank: dataRank)
    Data values to be used as the NeXus *plottable data*. As the upper case ``DATA ...
    @signal: (optional) [NX_POSINT](https://manual.nexusformat.org/nxdl-types.html#nx-posint)
        Plottable (independent) axis, indicate index number. ...
    @axes: (optional) [NX_CHAR](https://manual.nexusformat.org/nxdl-types.html#nx-char)
        Defines the names of the coordinates ...
    @long_name: (optional) [NX_CHAR](https://manual.nexusformat.org/nxdl-types.html#nx-char)
        data label

[rayosborn]

I believe that the designation of optional for the signal and axes attributes is a mistake, although it's possible I should lower the log message level to warning. It might have been left in when we were transitioning from the now-deprecated method of adding these attributes to their respective fields. In the case of the axes attribute, it could also be a result of the confusion that followed the introduction of the AXISNAME_indices attributes, which could, in principle, have replaced the axes attribute. However, at the recent NIAC, it was decided that the axes attribute was the primary way of designating the order of the axes. Whether it is strictly mandatory, I am not sure. I will raise this with the NIAC.

I would recommend adding both these attributes to all NXdata groups if at all possible.