I think it would be good, and useful, if it was possible to do a graph, with 
the y value defined as a function of x. The user gives the range of x values 
that s/he wants to graph.
This feature would be most (only?) useful in an XY graph.

How I envision this working, is if you select two cells, (e.g. A1 and B1), A1 
will contain a constant, and B1 will contain a function of A1 (e.g. =A1*2+4).

A dialogue will ask the user for the min or max values of x (A1) to graph. 
Maybe with an 'auto' checkbox too.

Clicking "Create" will then make an X-Y graph of this formula.

I realise that one can graph this manually, by Filling A1 and B1 downwards, 
then graphing the resulting values, but I don't think it's as flexible.

This feature would be useful for Maths, also economics and other disciplines, 
it acts as a graphical substitute for Goal Seek, (and it can find maxima and 
minima on a graph).

thanks :)