It would be nice if I could create a circle using area as a value (in addition to what we have now). It would work like this

1) Type the area in pixels
2) Give the aspect ratio

How this helps:

Case 1:
So if I want to create a diagram using circles to show that in 1900 world population was (suppose) 1.5 Billions, and in 2008 it is 6+ Billion, right now I will have to do the maths and calculate radius. It would be neat if Inkscape could do it for me. I just want to enter the area value 150 and 600 to create two circles.

Case 2:
Now if I wanted to show the comparison of size for moon and earth. I would enter the values for width and height (like I do now) to equal diameter of earth to create one circle. Inkscape could calculate and fill up the area too. Now earth is oblate. Suppose I knew by how much (through Wikipedia), I would change the aspect ratio from 1 to (say) 1.005 (width/height) to reflect the oblateness of earth in comparison of moon.

Or you could do your graphs in Gnumeric and export them as SVG