ALL software version info (bokeh, python, notebook, OS, browser, any other relevant packages)

python: 3.8.5
bokeh: 2.2.1
jupyter_core: 4.6.3
jupyter_client: 6.1.6
windows 10 Home

Description of expected behavior and the observed behavior

Slope does not draw any line when gradient == 0. I would expect it to draw a horizontal line. Seem funky to have to check if my gradient is

https://readthedocs.org/projects/chartify/builds/

When plotting with gnuplot on windows, Helvetica is not available by default. Would it be ok to use alternative on Windows?

.\example_plot_1.exe

(process:23384): Pango-WARNING **: 17:16:06.198: couldn't load font "Helvetica Not-Rotated 200", falling back to "Sans Not-Rotated 200", expect ugly output.

I believe the API is stable enough to begin documenting in roxygen and also in other supplementary materials, such as vignettes and articles.

For instance should it be called Mortality Rate, or Known Mortality Rate? Each of the labels should have a tooltip perhaps explaining in a bit more detail, but every page should list not only sources, but how we arrived at the data (or link to a "Methodology" page)