Yes. Or possibly more correctly:

=if(isblank(B45),NA(),(B45-32)*5/9)

as isblank is the "proper" function for checking a cell is empty.

Just thought I'd post a solution I've just written.

To recap: to get the graph to appear as I want I mustn't have anything in the Centigrade cells, but when I enter a temperature in the Fahrenheit cell I want the corresponding Centigrade cell to display the appropriate value.

I've written a tiny VBA routine utilising the Worksheet_Change event to detect any non-blank Fahrenheit cells and to ensure the corresponding Centigrade cell contains the conversion formula. And to allow for deleting a Fahrenheit value it checks that the Centigrade cell corresponding to a blank Fahrenheit cell is also blank.

All seems to work fine.

The only difference I've noticed is that after inserting the Fahrenheit value - and the Centigrade value being automatically planted - the Undo function isn't available. Seems reasonable as the last thing actually done was the VBA routine.