Altair saver ValueError：不支持的格式：'png'

Question

当我尝试在 Jupyter Notebook 中将图表保存为 PNG 时，使用 Altair saver 时不断出现错误。ValueError: Unsupported format: 'png'

我从这里知道我需要设置渲染器启用，从这里知道自述文件中有一个拼写错误，所以我在第五行代码中得到了正确的结果。

运行以下命令：
Windows 10
conda 4.8.2
Python 3.8.3
altair 4.1.0 py_1 conda-forge
altair_saver 0.1.0 py_0 conda-forge
vega 3.4.0 py38h32f6830_0 conda-forge
selenium 3.141.0 py38h9de7a3e_1001 conda-forge

import pandas as pd
import altair as alt
from altair_saver import save
alt.renderers.enable('default'); # if in jupyter, ; to suppress output
alt.renderers.enable('altair_saver', fmts=['vega-lite', 'png']);

mytaskbars = pd.DataFrame([
    {"task": "Task1a", "start": '2020-06-01', "end": '2020-09-30', "color": 'royalblue'},
    {"task": "Task1b", "start": '2020-06-01', "end": '2021-03-31', "color": 'deepskyblue'},
    {"task": "Task2", "start": '2020-06-01', "end": '2021-03-31', "color": 'red'},
    ])
    
mytaskbars["start"] = pd.to_datetime(mytaskbars["start"])
mytaskbars["end"] = pd.to_datetime(mytaskbars["end"])
    
    
chart = alt.Chart(mytaskbars).mark_bar(opacity=0.7).encode(
    x=alt.X('start', axis=alt.Axis(title='Date', labelAngle=-45, format = ("%b %Y"))),
     x2 = 'end',
    y=alt.Y('task', axis=alt.Axis(title=None)),
    color = alt.Color('color:N', scale = None)
    )
    
save(chart, "chart_202006.png")
chart

我也按照这里的chart.save('chart_202006.png')指示尝试过，同样的错误。我认为该错误在某种程度上与渲染器行相关，即使该错误是稍后抛出的。

这是完整的回溯，按照评论中的要求：

ValueError                                Traceback (most recent call last)
<ipython-input-3-13a284c2aca9> in <module>
     19     )
     20 
---> 21 save(chart, "chart_202006.png")
     22 chart

~\anaconda3\envs\geospat_env\lib\site-packages\altair_saver\_core.py in save(chart, fp, fmt, mode, method, **kwargs)
     60     """
     61     if method is None:
---> 62         Saver = _get_saver_for_format(fmt=fmt, fp=fp)
     63     elif isinstance(method, type):
     64         Saver = method

~\anaconda3\envs\geospat_env\lib\site-packages\altair_saver\_core.py in _get_saver_for_format(fmt, fp)
     28         if fmt in s.valid_formats and s.enabled():
     29             return s
---> 30     raise ValueError(f"Unsupported format: {fmt!r}")
     31 
     32 

ValueError: Unsupported format: 'png'

Answer 1

任何寻找快速复制粘贴解决方案的人：

$ pip install  altair_saver
$ npm install vega-lite vega-cli canvas