ENH: Styler.bar extended to allow centering about the mean, value or callable

doc/source/user_guide/style.ipynb

@@ -1190,9 +1190,9 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "In version 0.20.0 the ability to customize the bar chart further was given. You can now have the `df.style.bar` be centered on zero or midpoint value (in addition to the already existing way of having the min value at the left side of the cell), and you can pass a list of `[color_negative, color_positive]`.\n",
+    "Additional keyword arguments give more control on centering and positioning, and you can pass a list of `[color_negative, color_positive]` to highlight lower and higher values.\n",
     "\n",
-    "Here's how you can change the above with the new `align='mid'` option:"
+    "Here's how you can change the above with the new `align` option, combined with setting `vmin` and `vmax` limits, the `width` of the figure, and underlying css `props` of cells, leaving space to display the text and the bars:"
    ]
   },
   {
@@ -1201,7 +1201,8 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "df2.style.bar(subset=['A', 'B'], align='mid', color=['#d65f5f', '#5fba7d'])"
+    "df2.style.bar(align=0, vmin=-2.5, vmax=2.5, color=['#d65f5f', '#5fba7d'],\n",
+    "              width=60, props=\"width: 120px; border-right: 1px solid black;\").format('{:.3f}', na_rep=\"\")"
    ]
   },
   {
@@ -1225,28 +1226,31 @@
     "\n",
     "# Test series\n",
     "test1 = pd.Series([-100,-60,-30,-20], name='All Negative')\n",
-    "test2 = pd.Series([10,20,50,100], name='All Positive')\n",
-    "test3 = pd.Series([-10,-5,0,90], name='Both Pos and Neg')\n",
+    "test2 = pd.Series([-10,-5,0,90], name='Both Pos and Neg')\n",
+    "test3 = pd.Series([10,20,50,100], name='All Positive')\n",
+    "test4 = pd.Series([100, 103, 101, 102], name='Large Positive')\n",
+    "\n",
     "\n",
     "head = \"\"\"\n",
     "<table>\n",
     "    <thead>\n",
     "        <th>Align</th>\n",
     "        <th>All Negative</th>\n",
-    "        <th>All Positive</th>\n",
     "        <th>Both Neg and Pos</th>\n",
+    "        <th>All Positive</th>\n",
+    "        <th>Large Positive</th>\n",
     "    </thead>\n",
     "    </tbody>\n",
     "\n",
     "\"\"\"\n",
     "\n",
-    "aligns = ['left','zero','mid']\n",
+    "aligns = ['left', 'right', 'zero', 'mid', 'mean', 99]\n",
     "for align in aligns:\n",
     "    row = \"<tr><th>{}</th>\".format(align)\n",
-    "    for series in [test1,test2,test3]:\n",
+    "    for series in [test1,test2,test3, test4]:\n",
     "        s = series.copy()\n",
     "        s.name=''\n",
-    "        row += \"<td>{}</td>\".format(s.to_frame().style.bar(align=align, \n",
+    "        row += \"<td>{}</td>\".format(s.to_frame().style.hide_index().bar(align=align, \n",
     "                                                           color=['#d65f5f', '#5fba7d'], \n",
     "                                                           width=100).render()) #testn['width']\n",
     "    row += '</tr>'\n",

doc/source/whatsnew/v1.4.0.rst

@@ -30,6 +30,7 @@ enhancement2
 Other enhancements
 ^^^^^^^^^^^^^^^^^^
 - :meth:`Series.sample`, :meth:`DataFrame.sample`, and :meth:`.GroupBy.sample` now accept a ``np.random.Generator`` as input to ``random_state``. A generator will be more performant, especially with ``replace=False`` (:issue:`38100`)
+-  Additional options added to :meth:`.Styler.bar` to control alignment and display (:issue:`26070`)
 - :meth:`Series.ewm`, :meth:`DataFrame.ewm`, now support a ``method`` argument with a ``'table'`` option that performs the windowing operation over an entire :class:`DataFrame`. See :ref:`Window Overview <window.overview>` for performance and functional benefits (:issue:`42273`)
 -
 

pandas/io/formats/style.py

@@ -2042,75 +2042,16 @@ def set_properties(self, subset: Subset | None = None, **kwargs) -> Styler:
         values = "".join(f"{p}: {v};" for p, v in kwargs.items())
         return self.applymap(lambda x: values, subset=subset)
 
-    @staticmethod
-    def _bar(
-        s,
-        align: str,
-        colors: list[str],
-        width: float = 100,
-        vmin: float | None = None,
-        vmax: float | None = None,
-    ):
-        """
-        Draw bar chart in dataframe cells.
-        """
-        # Get input value range.
-        smin = np.nanmin(s.to_numpy()) if vmin is None else vmin
-        smax = np.nanmax(s.to_numpy()) if vmax is None else vmax
-        if align == "mid":
-            smin = min(0, smin)
-            smax = max(0, smax)
-        elif align == "zero":
-            # For "zero" mode, we want the range to be symmetrical around zero.
-            smax = max(abs(smin), abs(smax))
-            smin = -smax
-        # Transform to percent-range of linear-gradient
-        normed = width * (s.to_numpy(dtype=float) - smin) / (smax - smin + 1e-12)
-        zero = -width * smin / (smax - smin + 1e-12)
-
-        def css_bar(start: float, end: float, color: str) -> str:
-            """
-            Generate CSS code to draw a bar from start to end.
-            """
-            css = "width: 10em; height: 80%;"
-            if end > start:
-                css += "background: linear-gradient(90deg,"
-                if start > 0:
-                    css += f" transparent {start:.1f}%, {color} {start:.1f}%, "
-                e = min(end, width)
-                css += f"{color} {e:.1f}%, transparent {e:.1f}%)"
-            return css
-
-        def css(x):
-            if pd.isna(x):
-                return ""
-
-            # avoid deprecated indexing `colors[x > zero]`
-            color = colors[1] if x > zero else colors[0]
-
-            if align == "left":
-                return css_bar(0, x, color)
-            else:
-                return css_bar(min(x, zero), max(x, zero), color)
-
-        if s.ndim == 1:
-            return [css(x) for x in normed]
-        else:
-            return DataFrame(
-                [[css(x) for x in row] for row in normed],
-                index=s.index,
-                columns=s.columns,
-            )
-
     def bar(
         self,
         subset: Subset | None = None,
         axis: Axis | None = 0,
         color="#d65f5f",
         width: float = 100,
-        align: str = "left",
+        align: str | float | int | Callable = "mid",
         vmin: float | None = None,
         vmax: float | None = None,
+        props: str = "width: 10em;",
     ) -> Styler:
         """
         Draw bar chart in the cell backgrounds.
@@ -2131,16 +2072,26 @@ def bar(
             first element is the color_negative and the second is the
             color_positive (eg: ['#d65f5f', '#5fba7d']).
         width : float, default 100
-            A number between 0 or 100. The largest value will cover `width`
-            percent of the cell's width.
-        align : {'left', 'zero',' mid'}, default 'left'
-            How to align the bars with the cells.
-
-            - 'left' : the min value starts at the left of the cell.
+            The percentage of the cell, measured from the left, in which to draw the
+            bars, in [0, 100].
+        align : str, int, float, callable, default 'mid'
+            How to align the bars within the cells relative to a width adjusted center.
+            If string must be one of:
+
+            - 'left' : bars are drawn rightwards from the minimum data value.
+            - 'right' : bars are drawn leftwards from the maximum data value.
             - 'zero' : a value of zero is located at the center of the cell.
-            - 'mid' : the center of the cell is at (max-min)/2, or
-              if values are all negative (positive) the zero is aligned
-              at the right (left) of the cell.
+            - 'mid' : a value of (max-min)/2 is located at the center of the cell,
+              or if all values are negative (positive) the zero is
+              aligned at the right (left) of the cell.
+            - 'mean' : the mean value of the data is located at the center of the cell.
+
+            If a float or integer is given this will indicate the center of the cell.
+
+            If a callable should take a 1d or 2d array and return a scalar.
+
+            .. versionchanged:: 1.4.0
+
         vmin : float, optional
             Minimum bar value, defining the left hand limit
             of the bar drawing range, lower values are clipped to `vmin`.
@@ -2149,14 +2100,16 @@ def bar(
             Maximum bar value, defining the right hand limit
             of the bar drawing range, higher values are clipped to `vmax`.
             When None (default): the maximum value of the data will be used.
+        props : str, optional
+            The base CSS of the cell that is extended to add the bar chart. Defaults to
+            `"width: 10em;"`
+
+            .. versionadded:: 1.4.0
 
         Returns
         -------
         self : Styler
         """
-        if align not in ("left", "zero", "mid"):
-            raise ValueError("`align` must be one of {'left', 'zero',' mid'}")
-
         if not (is_list_like(color)):
             color = [color, color]
         elif len(color) == 1:
@@ -2172,14 +2125,15 @@ def bar(
             subset = self.data.select_dtypes(include=np.number).columns
 
         self.apply(
-            self._bar,
+            _bar,
             subset=subset,
             axis=axis,
             align=align,
             colors=color,
-            width=width,
+            width=width / 100,
             vmin=vmin,
             vmax=vmax,
+            base_css=props,
         )
 
         return self
@@ -2830,3 +2784,166 @@ def _highlight_between(
         else np.full(data.shape, True, dtype=bool)
     )
     return np.where(g_left & l_right, props, "")
+
+
+def _bar(
+    data: FrameOrSeries,
+    align: str | float | int | Callable,
+    colors: list[str],
+    width: float,
+    vmin: float | None,
+    vmax: float | None,
+    base_css: str,
+):
+    """
+    Draw bar chart in data cells using HTML CSS linear gradient.
+
+    Parameters
+    ----------
+    data : Series or DataFrame
+        Underling subset of Styler data on which operations are performed.
+    align : str in {"left", "right", "mid", "zero", "mean"}, int, float, callable
+        Method for how bars are structured or scalar value of centre point.
+    colors : list-like of str
+        Two listed colors as string in valid CSS.
+    width : float in [0,1]
+        The percentage of the cell, measured from left, where drawn bars will reside.
+    vmin : float, optional
+        Overwrite the minimum value of the window.
+    vmax : float, optional
+        Overwrite the maximum value of the window.
+    base_css : str
+        Additional CSS that is included in the cell before bars are drawn.
+    """
+
+    def css_bar(start: float, end: float, color: str) -> str:
+        """
+        Generate CSS code to draw a bar from start to end in a table cell.
+
+        Uses linear-gradient.
+
+        Parameters
+        ----------
+        start : float
+            Relative positional start of bar coloring in [0,1]
+        end : float
+            Relative positional end of the bar coloring in [0,1]
+        color : str
+            CSS valid color to apply.
+
+        Returns
+        -------
+        str : The CSS applicable to the cell.
+
+        Notes
+        -----
+        Uses ``base_css`` from outer scope.
+        """
+        cell_css = base_css
+        if end > start:
+            cell_css += "background: linear-gradient(90deg,"
+            if start > 0:
+                cell_css += f" transparent {start*100:.1f}%, {color} {start*100:.1f}%,"
+            cell_css += f" {color} {end*100:.1f}%, transparent {end*100:.1f}%)"
+        return cell_css
+
+    def css_calc(x, left: float, right: float, align: str):
+        """
+        Return the correct CSS for bar placement based on calculated values.
+
+        Parameters
+        ----------
+        x : float
+            Value which determines the bar placement.
+        left : float
+            Value marking the left side of calculation, usually minimum of data.
+        right : float
+            Value marking the right side of the calculation, usually maximum of data
+            (left < right).
+        align : {"left", "right", "zero", "mid"}
+            How the bars will be positioned.
+            "left", "right", "zero" can be used with any values for ``left``, ``right``.
+            "mid" can only be used where ``left <= 0`` and ``right >= 0``.
+            "zero" is used to specify a center when all values ``x``, ``left``,
+            ``right`` are translated, e.g. by say a mean or median.
+
+        Returns
+        -------
+        str : Resultant CSS with linear gradient.
+
+        Notes
+        -----
+        Uses ``colors`` and ``width`` from outer scope.
+        """
+        if pd.isna(x):
+            return base_css
+
+        color = colors[0] if x < 0 else colors[1]
+        x = left if x < left else x
+        x = right if x > right else x  # trim data if outside of the window
+
+        start: float = 0
+        end: float = 1
+
+        if align == "left":
+            # all proportions are measured from the left side between left and right
+            end = (x - left) / (right - left)
+
+        elif align == "right":
+            # all proportions are measured from the right side between left and right
+            start = (x - left) / (right - left)
+
+        else:
+            z_frac: float = 0.5  # location of zero based on the left-right range
+            if align == "zero":
+                # all proportions are measured from the center at zero
+                limit: float = max(abs(left), abs(right))
+                left, right = -limit, limit
+            elif align == "mid":
+                # bars drawn from zero either leftwards or rightwards with center at mid
+                mid: float = (left + right) / 2
+                z_frac = (
+                    -mid / (right - left) + 0.5 if mid < 0 else -left / (right - left)
+                )
+
+            if x < 0:
+                start, end = (x - left) / (right - left), z_frac
+            else:
+                start, end = z_frac, (x - left) / (right - left)
+
+        return css_bar(start * width, end * width, color)
+
+    values = data.to_numpy()
+    left = np.nanmin(values) if vmin is None else vmin
+    right = np.nanmax(values) if vmax is None else vmax
+    z: float = 0  # adjustment to translate data
+
+    if align == "mid":
+        if left >= 0:  # "mid" is documented to act as "left" if all values positive
+            align, left = "left", 0 if vmin is None else vmin
+        elif right <= 0:  # "mid" is documented to act as "right" if all values negative
+            align, right = "right", 0 if vmax is None else vmax
+    elif align == "mean":
+        z, align = np.nanmean(values), "zero"
+    elif callable(align):
+        z, align = align(values), "zero"
+    elif isinstance(align, (float, int)):
+        z, align = float(align), "zero"
+    elif not (align == "left" or align == "right" or align == "zero"):
+        raise ValueError(
+            "`align` should be in {'left', 'right', 'mid', 'mean', 'zero'} or be a "
+            "value defining the center line or a callable that returns a float"
+        )
+
+    assert isinstance(align, str)  # mypy: should now be in [left, right, mid, zero]
+    if data.ndim == 1:
+        return [css_calc(x - z, left - z, right - z, align) for x in values]
+    else:
+        return DataFrame(
+            [
+                [css_calc(x - z, left - z, right - z, align) for x in row]
+                for row in values
+            ],
+            index=data.index,
+            columns=data.columns,
+        )