Spline interpolation in charts.

[Jean Bréfort <jean brefort normalesup org>]

Hi,

I just implemented various spline interpolation modes in charts:
- Bezier splines (which replace the previous code);
- Closed Bezier splines;
- Cubic splines with natural, parabolic, and cubic limits.

Cubic splines are not available for radial plots, and need strictly
increasing x values. When the x values are evenly spaced, the natural
cubic spline is identical to the Bezier spline. When x values are not
ordered, it would be possible to sort them, at least if there are not
duplicates, but I do not intend to do that for now, unless somebody
needs it with strong arguments.

Interpolations can also skip invalid data on request with one exception:
closed splines always skip invalid data.

Still to do: clamped cubic splines (needs some non trivial changes to
the API) and B-splines (no schedule).

Comments are welcome.

The code is in svn trunk.

Best regards,
Jean