Fixes to peek()

This PR is meant to address #1076. Also took the opportunity to try to normalize the plotting in lightcurves,
This is not a change you introduce, but do we really need to do:
```python
datetime.strptime(x + '/' + y, '%Y/%m')
```
could not we do just: 
```python
datetime.strptime(x + y, '%Y%m')
```
I think that if `x` and `y` are already strings so the `+` would not add but concatenate them.  In any case if they would not be strings that would fail.