使用Matplotlib绘制DataFrame与数组时遇InvalidIndexError求助

问题场景 #

• X_train是形状为(1343, 1)的pandas DataFrame
• y_pred_baseline是长度为1343的numpy数组
• 执行的绘图代码：

plt.plot(X_train, y_pred_baseline, color = "orange", label = "Baseline Model")

报错信息 #

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
File /opt/conda/lib/python3.9/site-packages/pandas/core/indexes/base.py:3629, in Index.get_loc(self, key, method, tolerance)
   3628 try:
-> 3629     return self._engine.get_loc(casted_key)
   3630 except KeyError as err:

File /opt/conda/lib/python3.9/site-packages/pandas/_libs/index.pyx:136, in pandas._libs.index.IndexEngine.get_loc()

File /opt/conda/lib/python3.9/site-packages/pandas/_libs/index.pyx:142, in pandas._libs.index.IndexEngine.get_loc()

TypeError: '(slice(None, None, None), None)' is an invalid key

During handling of the above exception, another exception occurred:

InvalidIndexError                         Traceback (most recent call last)
Cell In [148], line 1
----> 1 plt.plot(X_train, y_pred_baseline, color = "orange", label = "Baseline Model")
      2 plt.scatter(X_train, y_train)
      3 plt.xlabel("Area [sq meters]")

File /opt/conda/lib/python3.9/site-packages/matplotlib/pyplot.py:3019, in plot(scalex, scaley, data, *args, **kwargs)
   3017 @_copy_docstring_and_deprecators(Axes.plot)
   3018 def plot(*args, scalex=True, scaley=True, data=None, **kwargs):
-> 3019     return gca().plot(
   3020         *args, scalex=scalex, scaley=scaley,
   3021         **({"data": data} if data is not None else {}), **kwargs)

File /opt/conda/lib/python3.9/site-packages/matplotlib/axes/_axes.py:1605, in Axes.plot(self, scalex, scaley, data, *args, **kwargs)
   1363 """
   1364 Plot y versus x as lines and/or markers.
   1365 
   (...)
   1602 (``'green'``) or hex strings (``'#008000'``).
   1603 """
   1604 kwargs = cbook.normalize_kwargs(kwargs, mlines.Line2D)
-> 1605 lines = [*self._get_lines(*args, data=data, **kwargs)]
   1606 for line in lines:
   1607     self.add_line(line)

File /opt/conda/lib/python3.9/site-packages/matplotlib/axes/_base.py:315, in _process_plot_var_args.__call__(self, data, *args, **kwargs)
    313     this += args[0],
    314     args = args[1:]
-> 315 yield from self._plot_args(this, kwargs)

File /opt/conda/lib/python3.9/site-packages/matplotlib/axes/_base.py:490, in _process_plot_var_args._plot_args(self, tup, kwargs, return_kwargs)
    487         kw[prop_name] = val
    489 if len(xy) == 2:
-> 490     x = _check_1d(xy[0])
    491     y = _check_1d(xy[1])
    492 else:

File /opt/conda/lib/python3.9/site-packages/matplotlib/cbook/__init__.py:1362, in _check_1d(x)
   1356 with warnings.catch_warnings(record=True) as w:
   1357     warnings.filterwarnings(
   1358         "always",
   1359         category=Warning,
   1360         message='Support for multi-dimensional indexing')
-> 1362     ndim = x[:, None].ndim
   1363     # we have definitely hit a pandas index or series object
   1364     # cast to a numpy array.
   1365     if len(w) > 0:

File /opt/conda/lib/python3.9/site-packages/pandas/core/frame.py:3505, in DataFrame.__getitem__(self, key)
   3503 if self.columns.nlevels > 1:
   3504     return self._getitem_multilevel(key)
-> 3505 indexer = self.columns.get_loc(key)
   3506 if is_integer(indexer):
   3507     indexer = [indexer]

File /opt/conda/lib/python3.9/site-packages/pandas/core/indexes/base.py:3636, in Index.get_loc(self, key, method, tolerance)
   3631         raise KeyError(key) from err
   3632     except TypeError:
   3633         # If we have a listlike key, _check_indexing_error will raise
   3634         #  InvalidIndexError. Otherwise we fall through and re-raise
   3635         #  the TypeError.
-> 3636         self._check_indexing_error(key)
   3637         raise
   3639 # GH#42269

File /opt/conda/lib/python3.9/site-packages/pandas/core/indexes/base.py:5651, in Index._check_indexing_error(self, key)
   5647 def _check_indexing_error(self, key):
   5648     if not is_scalar(key):
   5649         # if key is not a scalar, directly raise an error (the code below
   5650         # would convert to numpy arrays and raise later any way) - GH29926
-> 5651         raise InvalidIndexError(key)

InvalidIndexError: (slice(None, None, None), None)

问题原因 #

Matplotlib的plot函数会调用_check_1d方法验证数据维度，该方法尝试用x[:, None]这种numpy风格的二维索引访问输入数据。但X_train是单列DataFrame，属于二维表格结构，pandas不支持这种索引方式，因此抛出InvalidIndexError。

Matplotlib要求x轴输入为一维序列（比如pandas Series或numpy数组），而非二维的DataFrame。

解决方法 #

方法1：提取DataFrame的单列转为Series #

通过索引提取单列，得到一维的Series对象：

# 方式a：通过位置索引提取
plt.plot(X_train.iloc[:, 0], y_pred_baseline, color = "orange", label = "Baseline Model")

# 方式b：通过列名提取（若知道列名，比如列名为'Area'）
plt.plot(X_train['Area'], y_pred_baseline, color = "orange", label = "Baseline Model")

方法2：将DataFrame转为numpy数组 #

直接将DataFrame转换为一维numpy数组：

plt.plot(X_train.to_numpy(), y_pred_baseline, color = "orange", label = "Baseline Model")
# 或者使用values属性（to_numpy()是更推荐的方式）
plt.plot(X_train.values, y_pred_baseline, color = "orange", label = "Baseline Model")

内容的提问来源于stack exchange，提问作者Kazien