ConstantLine( ) doesn't appear to work with date values within TimePlot( )

Thanks for the BeakerX package - very nice!

In a Jupyter notebook (in Python/pandas) I am trying to plot a constant line on a `TimePlot()` and it doesn't seem to recognise the date as a date - it is not clear to me how it is being handled. If I just plot a two point series/dataframe to replicate a constant (vertical) line it works fine - but passing in the `x=DateValue` doesn't. I will create a simple example and attach.