Documented why PySide2 support is still experimental

Author: PierreRaybaut

README.md

@@ -81,10 +81,46 @@ for more details on API limitations when comparing to Qwt.
 
 ### Requirements ###
 - Python >=2.6 or Python >=3.2
-- PyQt4 >=4.4 or PyQt5 >= 5.5
+- PyQt4 >=4.4 or PyQt5 >= 5.5 (or PySide2, still experimental, see below)
 - QtPy >= 1.3
 - NumPy >= 1.5
 
+### Why PySide2 support is still experimental ###
+
+<img src="https://raw.githubusercontent.com/PierreRaybaut/PythonQwt/master/doc/images/pyqt5_vs_pyside2.png">
+
+Try running the `curvebenchmark1.py` test with PyQt5 and PySide: you will notice a 
+huge performance issue with PySide2 (see screenshot above). This is due to the fact 
+that PyQt5 (and PyQt4) allows an efficient way of filling a QPolygonF object from a 
+Numpy array, and PySide2 is not (see function `qwt.plot_curve.series_to_polyline` 
+below).
+
+```python
+def series_to_polyline(xMap, yMap, series, from_, to):
+    """
+    Convert series data to QPolygon(F) polyline
+    """
+    xData = xMap.transform(series.xData()[from_ : to + 1])
+    yData = yMap.transform(series.yData()[from_ : to + 1])
+    size = to - from_ + 1
+    if PYSIDE2:
+        polyline = QPolygonF()
+        for index in range(size):
+            polyline.append(QPointF(xData[index], yData[index]))
+    else:
+        polyline = QPolygonF(size)
+        pointer = polyline.data()
+        dtype, tinfo = np.float, np.finfo  # integers: = np.int, np.iinfo
+        pointer.setsize(2 * polyline.size() * tinfo(dtype).dtype.itemsize)
+        memory = np.frombuffer(pointer, dtype)
+        memory[: (to - from_) * 2 + 1 : 2] = xData
+        memory[1 : (to - from_) * 2 + 2 : 2] = yData
+    return polyline
+```
+
+As a consequence, until an equivalent feature is implemented in PySide2, we strongly 
+recommend using PyQt5 instead of PySide2.
+
 ## Installation
 
 From the source package:

doc/images/pyqt5_vs_pyside2.png

@@ -6,7 +6,7 @@ Dependencies
 
 Requirements:
     * Python 2.x (x>=6) or 3.x (x>=2)
-    * PyQt4 4.x (x>=3 ; recommended x>=4) or PyQt5 5.x (x>=5)
+    * PyQt4 4.x (x>=4) or PyQt5 5.x (x>=5) or PySide2 (still experimental, see below)
     * QtPy >= 1.3
     * NumPy 1.x (x>=5)
     * Sphinx 1.x (x>=1) for documentation generation
@@ -18,6 +18,22 @@ From the source package:
 
     `python setup.py install`
 
+Why PySide2 support is still experimental
+-----------------------------------------
+
+.. image:: /images/pyqt5_vs_pyside2.png
+
+Try running the `curvebenchmark1.py` test with PyQt5 and PySide: you will notice a 
+huge performance issue with PySide2 (see screenshot above). This is due to the fact 
+that PyQt5 (and PyQt4) allows an efficient way of filling a QPolygonF object from a 
+Numpy array, and PySide2 is not (see code below).
+
+.. literalinclude:: /../qwt/plot_curve.py
+   :pyobject: series_to_polyline
+
+As a consequence, until an equivalent feature is implemented in PySide2, we strongly 
+recommend using PyQt5 instead of PySide2.
+
 Help and support
 ----------------