Numbers x-axis labels = 'untitled' ... (argh)

Apologies if this has been posted before. Am struggling to get a hold of Numbers axis labelling for new data entry.

I currently have a 2D line chart with x & y axis data plotted 100% accurately, but only up to a certain point.

Any new entries I add the x axis label name displays as 'Untitled'? The y axis plots the new data correctly.

The x axis is a range of numerical dates (eg 01/01/18). Entries 1-77 are ok. Entry 78 onwards are all 'Untitled'?

I can manually double click on the x axis range and manually edit the labels to show the correct dates, but this is not what I want at all. I would have thought Numbers could automatically insert the newer entries and labels once the data references had been updated?

As I say y axis works fine. The data references are correct - highlighted red and blue to the correct position. It's just the x axis seemingly will not update

Am a bit lost!