The bokeh.charts API was deprecated and removed in 2017, and should not be used for anything at this point.
Since then, Bokeh support for categorical and bar plots in the well-supported bokeh.plotting API has gotten immensely better. It is now possible to rotate the axis labels on a bar chart in the standard way:
from math import pi
from bokeh.plotting import figure, show
types = list("ABCDE")
values = [10, 20, 50, 25, 35]
p = figure(x_range=types, plot_height=250)
p.vbar(x=types, top=values, width=0.9)
p.xaxis.major_label_orientation = pi/4 # standard styling code
show(p)

You can find more information and examples in the Handling Categorical Data section of the User's Guide.