Add line profile plugin back in.

I saved a copy of the line profile plugin in a branch and then deleted the plugin from the main qtmpl-viewer branch. Unfortunately, I forgot that rebasing off the main branch would erase the plugin. This commit just adds the plugin back.

skimage/viewer/plugins/plotplugin.py

@@ -0,0 +1,50 @@
+import numpy as np
+from PyQt4 import QtGui
+
+import matplotlib.pyplot as plt
+
+from ..utils import MatplotlibCanvas
+from .base import Plugin
+
+
+class PlotCanvas(MatplotlibCanvas):
+    """Canvas for displaying images.
+
+    This canvas derives from Matplotlib, and has attributes `fig` and `ax`,
+    which point to Matplotlib figure and axes.
+    """
+    def __init__(self, parent, height, width, **kwargs):
+        self.fig, self.ax = plt.subplots(figsize=(height, width), **kwargs)
+        super(PlotCanvas, self).__init__(parent, self.fig, **kwargs)
+        self.setMinimumHeight(150)
+
+class PlotPlugin(Plugin):
+    """Plugin for ImageViewer that contains a plot canvas.
+
+    Base class for plugins that contain a Matplotlib plot canvas, which can,
+    for example, display an image histogram.
+
+    See base Plugin class for additional details.
+    """
+
+    def attach(self, image_viewer):
+        super(PlotPlugin, self).attach(image_viewer)
+        # Add plot for displaying intensity profile.
+        self.add_plot()
+
+    def redraw(self):
+        """Redraw plot."""
+        self.canvas.draw_idle()
+
+    def add_plot(self, height=4, width=4):
+        self.canvas = PlotCanvas(self, height, width)
+        self.fig = self.canvas.fig
+        #TODO: Converted color is slightly different than Qt background.
+        qpalette = QtGui.QPalette()
+        qcolor = qpalette.color(QtGui.QPalette.Window)
+        bgcolor = qcolor.toRgb().value()
+        if np.isscalar(bgcolor):
+            bgcolor = str(bgcolor / 255.)
+        self.fig.patch.set_facecolor(bgcolor)
+        self.ax = self.canvas.ax
+        self.layout.addWidget(self.canvas, self.row, 0)