Squashed commit of the following:

commit f537627
Author: Boyuan Lu <blu38@wisc.edu>
Date:   Sat Apr 22 17:37:42 2023 -0500

    Add feature: flexible sector (and documentation)

commit f6e884d
Author: Boyuan Lu <blu38@wisc.edu>
Date:   Fri Apr 21 18:30:21 2023 -0500

    Add feature: flexible sector

windrose/windrose.py

@@ -569,6 +569,11 @@ def bar(self, direction, var, **kwargs):
             number of sectors used to compute the windrose table. If not set,
             nsectors=16, then each sector will be 360/16=22.5°, and the
             resulting computed table will be aligned with the cardinals points.
+        sectoroffset: float, optional
+            the offset for the sectors. By default, the offsect is zero, and
+            the first sector is [-360/nsector, 360/nsector] or [-11.25, 11.25]
+            for nsector=16. If offset is non-zero, the first sector will be
+            [-360/nsector + offset, 360/nsector + offset] and etc.
         bins : 1D array or integer, optional
             number of bins, or a sequence of bins variable. If not set, bins=6
             between min(`var`) and max(`var`).
@@ -611,6 +616,9 @@ def bar(self, direction, var, **kwargs):
 
         offs = self._calm_circle()
 
+        # sector offset in radius
+        sector_offset = kwargs.pop("sectoroffset", 0) / 180 * np.pi
+
         for j in range(nsector):
             offset = offs
             for i in range(nbins):
@@ -619,7 +627,7 @@ def bar(self, direction, var, **kwargs):
                 val = self._info["table"][i, j]
                 zorder = ZBASE + nbins - i
                 patch = mpl.patches.Rectangle(
-                    (angles[j] - opening / 2, offset),
+                    (angles[j] - opening / 2 - sector_offset, offset),
                     opening,
                     val,
                     facecolor=colors[i],
@@ -650,6 +658,11 @@ def box(self, direction, var, **kwargs):
             number of sectors used to compute the windrose table. If not set,
             nsectors=16, then each sector will be 360/16=22.5°, and the
             resulting computed table will be aligned with the cardinals points.
+        sectoroffset: float, optional
+            the offset for the sectors. By default, the offsect is zero, and
+            the first sector is [-360/nsector, 360/nsector] or [-11.25, 11.25]
+            for nsector=16. If offset is non-zero, the first sector will be
+            [-360/nsector + offset, 360/nsector + offset] and etc.
         bins : 1D array or integer, optional
             number of bins, or a sequence of bins variable. If not set, bins=6
             between min(`var`) and max(`var`).
@@ -685,6 +698,9 @@ def box(self, direction, var, **kwargs):
 
         offs = self._calm_circle()
 
+        # sector offset in radius
+        sector_offset = kwargs.pop("sectoroffset", 0) / 180 * np.pi
+
         for j in range(nsector):
             offset = offs
             for i in range(nbins):
@@ -693,7 +709,7 @@ def box(self, direction, var, **kwargs):
                 val = self._info["table"][i, j]
                 zorder = ZBASE + nbins - i
                 patch = mpl.patches.Rectangle(
-                    (angles[j] - opening[i] / 2, offset),
+                    (angles[j] - opening[i] / 2 - sector_offset, offset),
                     opening[i],
                     val,
                     facecolor=colors[i],
@@ -754,7 +770,8 @@ def pdf(
         return (self, params)
 
 
-def histogram(direction, var, bins, nsector, normed=False, blowto=False):
+def histogram(direction, var, bins, nsector, normed=False,
+              blowto=False, sectoroffset=0):
     """
     Returns an array where, for each sector of wind
     (centred on the north), we have the number of time the wind comes with a
@@ -770,7 +787,9 @@ def histogram(direction, var, bins, nsector, normed=False, blowto=False):
         list of var category against we're going to compute the table
     nsector : integer
         number of sectors
-
+    sectoroffset : float, default 0
+        sector offset. For example, the first sector is [-360/nsector+
+        sectoroffset, 360/nsector+sectoroffset], etc.
     Other Parameters
     ----------------
     normed : boolean, default False
@@ -785,7 +804,9 @@ def histogram(direction, var, bins, nsector, normed=False, blowto=False):
 
     angle = 360.0 / nsector
 
-    dir_bins = np.arange(-angle / 2, 360.0 + angle, angle, dtype=float)
+    dir_bins = np.arange(-angle / 2, 360.0 + angle, angle, dtype=float)\
+        + sectoroffset
+
     dir_edges = dir_bins.tolist()
     dir_edges.pop(-1)
     dir_edges[0] = dir_edges.pop(-1)