bokeh.plotting.helpers is gone in bokeh 2.0

Seems like they removed the entire bokeh.plotting.helpers in 2.0 release of bokeh, which causes an error in some interactive plotting that needs holoviews. Might want to pin it in the requirements to 1.4? (Great work on all the new vis options, by the way!)

Start by checking the project's dependency requirements for the bokeh.plotting.helpers issue affecting interactive plotting with holoviews. Confirm the failure with bokeh 2.0, then verify that the selected dependency constraint prevents the error and that the affected plotting use case works.

Written by the indexing model from the issue text.