Hi,

I (well actually a friend) was recently trying to fit a UnivariateSpline to some data and then evaluate the derivative. The spline generated fine and could be evaluated (it looked reasonable) but the derivative would give the error:

File "/usr/lib/pytho

It turned out that the x-array input was not in ascending order, as the documentation says is required. The exact data I was using lead to non-ascending knots.

I recognise this error is not encountered if the documentation is followed, but the error is uninformative, and it seems like an easy mistake to make. Especially given the spline still looks fine otherwise.

In case anyone wants to reproduce the example, the data are:

2.651,1.421
2.581,1.36
2.494,1.256
2.398,1.186
2.328,1.116
2.232,1.046
2.11,0.951
2.041,0.881
1.945,0.837
1.84,0.794
1.727,0.733
1.631,0.689
1.57,0.663
1.483,0.602
1.413,0.567
1.291,0.515
1.203,0.488
1.108,0.445
0.994,0.392
0.898,0.349
0.802,0.314
0.724,0.279
0.619,0.253
0.523,0.218
0.419,0.183
0.314,0.14
0.253,0.096
0.157,0.078
0.087,0.044