[Kst] [Bug 128483] UI: the "quick curve creation" part of theequation dialog should be clearly separated

[Brisset, Nicolas]

> The problem is that the 'x' vector serves two purposes:
> 
>   1) the X vector in the auto-created curve
>   2) the vector that replaces 'x' in equations (eg, when you 
> enter x^2)
I don't get this... For me, the "X vector" combobox was used to indicate
which X vector to use in the auto-created curve and that's all. If I
want to plot x^2 with x being, say, Vec1, then I'd simply write in the
equation lineedit: "[Vec1]^2". I don't  understand why we'd need
something like an auto-interpret x as ..., especially when there is only
one dialog item to select it. Sure, it is somewhat shorter to write but
not worth the confusion it creates as far as I'm concerned.
The only case where I imagine where this makes a significant usability
difference (in terms of number of mouse clicks/keystrokes) is to switch
from plotting [Vec1]^2 to [Vec2]^2 to [Vec3]^2 etc... via simple
mouseclick, without needing to edit the equation text in the lineedit.
But I don't quite see in which workflow this would be useful. And if it
really is useful for some people, then we could provide a plugin that
takes a f(x) string and a vector as input, and computes y=f(x) (quite
simple I think, at least with the muparser stuff that I used for the
nonlinear fit plugin but probably also possible without that
dependency). It would be redundant with the case of equations taking
only one X vector, but we can keep them, if only for cases where we use
more than one input vector (a case which by the way is not really
handled with the current approach: what if you compute one vector based
on two or more other vectors ?).

So, to sum up this rather long and confusing discussion, my suggestion
would be:
- implement the changes as I suggest in bugzilla
- provide a new plugin to compute y=f(x) taking a string (e.g. x^2) and
a vector as input
- keep equations as they are now for more than one input vector (or
users reluctant to use plugins)
- forget about the interpret x as... feature in equations (I wasn't even
aware it existed and it does not "scale" to multiple vectors anyway)

Does that sound acceptable ? I'd be willing to give a try at the plugin,
using the muparser stuff already in the non-linear fit plugin, unless
there is a preference for using kst "built-in" functionality to parse
the f(x) string and compute the output vector.

Nicolas