I discovered something some might find helpful. Suppose you have a chart
with references to an external worksheet, and you want to move the data to
the same sheet as the chart. Open both worksheets, select the data, CUT,
then paste to the chart's worksheet (using COPY and paste will not change the
chart's references to the pasted location). This method is far less tedious
than copying the data then changing the chart's source data.

If you don't want to lose the data in the original file, close it without
saving.

This will not work if formulas in the data refer to other cells. Unless the
formulas explicitly specify a worksheet, they will refer to the same cell
address in their new location (the chart worksheet), and those referenced
cells simply don't contain the same data. Here you will need to either copy
and paste VALUES, then re-source the chart (you would not have been able to
cut and paste values in the first place; special pasting is disabled for cut
clipboard data).