Bug in gradient evaluation near edges?

[tomasaschan]

Given the following,

using Interpolations
xs = -pi:2pi/20:pi
itp = interpolate(sin(xs), BSpline(Quadratic(Periodic)), OnGrid)

using Gadfly
plot(x=xs, y=[gradient(itp,x)[1] for x in xs], Geom.path)

I think we ought to be able to do better than this at the edges:

Just to make sure that this is related to edge-proximity, and not to the fact that the function tends to 0 in this specific case, I tested with the boundaries at [-3pi/4, 5pi/4] too, with similar results:

I'll see if I can do some troubleshooting, but I'm putting this up here to indicate that we at least know about the problem.

[tomasaschan]

Turns out this was a classic PEBKAC; in xs, the end point shouldn't be included (i.e. it works fine for xs = -pi:2pi/20:pi-2pi/20).