Merge pull request matplotlib#1010 from dmcdougall/offset_line

Port part of errorfill from Tony Yu's mpltools.

Author: mdboom

lib/matplotlib/mlab.py

@@ -3194,3 +3194,33 @@ def quad2cubic(q0x, q0y, q1x, q1y, q2x, q2y):
     c2x, c2y = c1x + 1./3. * (q2x - q0x), c1y + 1./3. * (q2y - q0y)
     # c3x, c3y = q2x, q2y
     return q0x, q0y, c1x, c1y, c2x, c2y, q2x, q2y
+
+def offset_line(y, yerr):
+    """
+    Offsets an array *y* by +/- an error and returns a tuple (y - err, y + err).
+
+    The error term can be:
+
+    o A scalar. In this case, the returned tuple is obvious.
+    o A vector of the same length as *y*. The quantities y +/- err are computed
+      component-wise.
+    o A tuple of length 2. In this case, yerr[0] is the error below *y* and
+      yerr[1] is error above *y*. For example::
+
+        from pylab import *
+        x = linspace(0, 2*pi, num=100, endpoint=True)
+        y = sin(x)
+        y_minus, y_plus = mlab.offset_line(y, 0.1)
+        plot(x, y)
+        fill_between(x, ym, y2=yp)
+        show()
+
+    """
+    if cbook.is_numlike(yerr) or (cbook.iterable(yerr) and len(yerr) == len(y)):
+        ymin = y - yerr
+        ymax = y + yerr
+    elif len(yerr) == 2:
+        ymin, ymax = y - yerr[0], y + yerr[1]
+    else:
+        raise ValueError("yerr must be scalar, 1xN or 2xN")
+    return ymin, ymax