Chart items disappear unless I retype data in cell manually

Sorry if the title sounds a bit obscure - the problem is a bit obscure, and quite difficult to explain.

I have a spreadsheet that contains data imported from a .txt file. I have created simple charts that display a portion of this data graphically. Until recently, each time I have added data to the table, I have been able to copy the chart, move the datum points and display the next day's data in a fresh copy of the chart.

But what is now happening is that, when I move the datum points by dragging the data markers (lassoes? what do you call those highlighting rectangles?) the points on the chart disappear - until I retype the data manually in the field! The figures look identical; the formatting is identical; but if I don't retype each item in one of the columns, the chart remains blank.

I have attached a small part of the spreadsheet as an example. If you open this and click on any of the 'blobs' on the chart, you will see the data markers appear on the left. The column E (which is simply a numerical representation of the Vote column) includes all of the entries for 1st November; the Time column includes only the first six entries.

If you now drag down the Time data marker to include the next item, all the chart entries disappear! Even more weirdly, although the data in the new cell is formatted exactly the same as the ones above it, retype it exactly as it is and abracadabra! - the chart symbols reappear, including the new one.

The second chart is based on the data for 2nd November. In this case I have only retyped the first item, but the result is the same.

Whilst I don't profess to being an Excel expert, I thought I was reasonably competent, but this one has me beaten!

Any ideas, please?

This may not be of much help but if you click on the cell and then either click F2 and Enter or click into the formula edit bar and hit enter it will fix without having to retype the data thus introducing opportunity to make an error. As to why that is necessary I don't know, it should not be.

You are right - I hadn't discovered that! But, as you say, it should not be necessary. Why is that happening, I wonder?

So - am I to assume that NOBODY out there has any idea why this is happening?

That's disappointing.

Thank you! That seems to solve the problem.

It would have been helpful if I could somehow have discovered how Excel was treating the data in the cells. I had thought that it might be something to do with formatting, so I formatted the column as TIME, but I guess the data was still stored internally as text. Is there any way of knowing how Excel is regarding data stored within individual cells - text, time, date, number etc?

Fascinating! But surely this highlights an odd glitch in Excel? Why would it return the 'correct' format even for those incorrect cells? And why is it necessary to carry out what, on the face of it, seems a fatuous assignment: Selection.Value = Selection.Value in order to make it right?? I don't think I am going to live long enough to understand all of the idiosyncrasies of this product.