Odd behaviour in Excel 2016 charts

I have a long running experiment with approx. 150 data points per day over approx. 3 years

Up until now I have been able to chart this in excel 2016 easily – by selecting the entire data range and selecting a line type chart – job done.
Moreover - if I wanted to look more closely at just sections of the timeline – I have been able to simply click on the horizontal (time) axis and in the format axis panel that opens on the right-hand side of the screen – manually edit the axis start date and end date.
See screenshot 1 ( you may need to open the image in a new window to see the format axis panel on the right)
This worked perfectly for my needs.

However, I have recently needed to reorder rows in the data set to clean them up and then redraw the chart, again by selecting the whole data set and selecting line chart.
However now I no longer have the option to manually edit the horizontal axis date range - and instead the format axis options look like this
See this screenshot ( I would post the spreadsheet – but its huge – and I have not been able to demonstrate the issue on other charts)
see screenshot 2
i have tried various combinations of options but don't seem to be able to get the option to alter the time axis start / finish dates manually
What am I missing?