create_trisurf raises an IndexError exception when one triangle's average height is very close to max height

When giving plotly.figure_factory.create_trisurf values where on triangle on the surface is very close to the maximum the function raises an IndexError. I'd expect a pretty plot instead.

Reproduction:

Python 3.6.9 (default, Jan 26 2021, 15:33:00) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import plotly
>>> import plotly.figure_factory as ff
>>> import numpy as np
>>> plotly.__version__
'5.1.0'
>>> vmin, vmax = -0.8, 0.2
>>> face = np.nextafter(vmax, -vmax) # the last value less than vmax
>>> # define a simple shape which includes 3 triangles of constant value vmin, vmax and face
... 
>>> x = [0.0, 0.0, 1.0, 1.0, 2.0, 2.0, 3.0, 3.0, 4.0]
>>> y = [1.0, 0.0, 1.0, 0.0, 1.0, 0.0, 1.0, 0.0, 1.0]
>>> z = [vmin, vmin, vmin, face, face, face, vmax, vmax, vmax]
>>> simplices = [[0, 1, 2], [1, 2, 3], [2, 3, 4], [3, 4, 5], [4, 5, 6], [5, 6, 7], [6, 7, 8]]
>>> ff.create_trisurf(x, y, z, simplices)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/.local/lib/python3.6/site-packages/plotly/figure_factory/_trisurf.py", line 488, in create_trisurf
    plot_edges=plot_edges,
  File "/home/user/.local/lib/python3.6/site-packages/plotly/figure_factory/_trisurf.py", line 153, in trisurf
    mean_dists[index], colormap, scale, min_mean_dists, max_mean_dists
  File "/home/user/.local/lib/python3.6/site-packages/plotly/figure_factory/_trisurf.py", line 48, in map_face2color
    colormap[low_color_index + 1],
IndexError: list index out of range

Also attached a file does the same thing, as a .txt file instead of .py because that's what github supports in this context.
plotly_trisurf_indexerror.txt

plotly/figure_factory/_trisurf.py から始め、特に map_face2color を確認してください。そこでは traceback が colormap への範囲外アクセスを報告しています。提供された Python と NumPy の再現コードを実行し、その後、create_trisurf が IndexError を発生させるのではなく正常に完了してプロットを生成することを確認してください。