hmm....you can actually test this (if matters I think you should). This is not free (perf wise), but I think makes sense in most/all? interpolations (you could do this before you pass to the interpolate_1d in the Block I think)

```
In [5]: Index([1,3,2,4]).is_monotonic
Out[5]: False

In [6]: Index([1,2,3,4]).is_monotonic
Out[6]: True
```