Vertical lines on TimePlot broken in python

The following code and variations of it produce a blank chart

```
y = pd.Series([7.5, 7.9, 7, 8.7, 8, 8.5])
dates = pd.Series(['2015-02-01',
'2015-02-02',
'2015-02-03',
'2015-02-04',
'2015-02-05',
'2015-02-06']
, dtype='datetime64[ns]')
plot = TimePlot()
plot.add(Line(x=dates, y=y))
plot.add(ConstantLine(x=pd.to_datetime('2015-02-04 15:00:00'))
```

If I switch to using nanoseconds as keys instead of Timestamp then it works as expected.
I'm attaching a couple of screenshots of the chart and the log with the error. I'm not even sure how to get to the backend log of beakerX in order to see what's the root exception, any direction would be very helpful.