Be more specific in typing x, y and z

Feature Description
It's a recurring problem to have x, y and z columns be sometimes undefined or unclear or nullable. We should separate this logic out somewhere and control it centrally instead of being all willy nilly.

        const axisDef = currGraph.axes[idx];
        if (!axisDef) return null;
        return {
          header: axisDef.axis,
          filename: `${fileKeys[0]}/${axisDef.file}`,
          timespan: { start: null, end: null }
        };
      });```

For example, instead of extracting this manually, write a helper function thats more in-depth and specific to the x, y, z approach.

Start by locating the dataColumnKeys mapping and the Column type shown in the issue, then trace how x, y, and z axis definitions are represented. Determine where the nullable and undefined cases are currently handled. Done means the x/y/z column construction is controlled by one specific helper with consistent types, but the issue does not name tests or a target file.