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

从 plotly/figure_factory/_trisurf.py 开始，重点查看 map_face2color，traceback 报告的正是该处对 colormap 的越界访问。运行所提供的 Python 和 NumPy 复现代码，然后验证 create_trisurf 能够完成并生成图表，而不是引发 IndexError。