added GradientWidget example

examples/GradientWidget.py

+# -*- coding: utf-8 -*-
+import initExample ## Add path to library (just for examples; you do not need this)
+
+import pyqtgraph as pg
+from pyqtgraph.Qt import QtCore, QtGui
+import numpy as np
+
+
+
+app = QtGui.QApplication([])
+w = QtGui.QMainWindow()
+w.show()
+w.resize(400,400)
+cw = QtGui.QWidget()
+w.setCentralWidget(cw)
+
+l = QtGui.QGridLayout()
+l.setSpacing(0)
+cw.setLayout(l)
+
+w1 = pg.GradientWidget(orientation='top')
+w2 = pg.GradientWidget(orientation='right', allowAdd=False)
+#w2.setTickColor(1, QtGui.QColor(255,255,255))
+w3 = pg.GradientWidget(orientation='bottom')
+w4 = pg.GradientWidget(orientation='left')
+w4.loadPreset('spectrum')
+label = QtGui.QLabel("""
+- Click a triangle to change its color
+- Drag triangles to move
+- Click in an empty area to add a new color
+    (adding is disabled for the right-side widget)
+- Right click a triangle to remove
+""")
+
+l.addWidget(w1, 0, 1)
+l.addWidget(w2, 1, 2)
+l.addWidget(w3, 2, 1)
+l.addWidget(w4, 1, 0)
+l.addWidget(label, 1, 1)
+
+
+## Start Qt event loop unless running in interactive mode or using pyside.
+import sys
+if (sys.flags.interactive != 1) or not hasattr(QtCore, 'PYQT_VERSION'):
+    QtGui.QApplication.instance().exec_()
+
+
+