Enter the formula in a worksheet cell
Activate the chart; type = in the formula bar; click on the cell with
formula
This give something like =Sheet1!D1, tap Enter key or clicks the formula
bar's green check mark
Now you have a text box in the chart and it displays the formula.
You can drag the textbox anywhere on the chart
best wishes
You can use Bernard's technique with chart and axis titles to make them
dynamic. In other words to have the chart title display the current date you
would single click the title, click on the formula bar, type = and then click
on the cell in the spreadsheet where the =TODAY() or =NOW() function was
located.