Fix bar with one value in it

[hoxbro]

Fixes #6300

A MRE:

import holoviews as hv
import pandas as pd

hv.extension("bokeh")

df = pd.DataFrame({"time": [1], "value": [-1]})
hv.Bars(df)

Need to add the MRE as a test.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.52%. Comparing base (febe075) to head (24f8da4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6301   +/-   ##
=======================================
  Coverage   88.52%   88.52%           
=======================================
  Files         323      323           
  Lines       67996    68007   +11     
=======================================
+ Hits        60193    60204   +11     
  Misses       7803     7803           

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[MarcSkovMadsen]

Thx