One very important feature (for me) is the possibility to easily moves graphs around without changing data in table.
That is to say, allowing options in graph to add a constant origin (x or y), as well as constant multiplicative factor.
I propose something like staying clicked on a curve for more than 2 seconds allows to move it around. And staying clicked more than 2 sec plus pressing Ctrl/Shift/Alt key allow to shift either in x direction only or y direction only. (very important feature to easily compare several graphs)
The actual value of this shifts could be accessed from a new "offset" tab in the "plot details" window.