Make Buffer following behavior toggleable (#3823)

examples/user_guide/16-Streaming_Data.ipynb

@@ -199,6 +199,13 @@
     "dfstream.clear()"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Note that when using the ``Buffer`` stream the view will always follow the current range of the data by default, by setting ``buffer.following=False`` or passing following as an argument to the constructor this behavior may be disabled."
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},

holoviews/plotting/bokeh/element.py

@@ -751,7 +751,8 @@ def _update_ranges(self, element, ranges):
         if any(isinstance(ax_range, FactorRange) for ax_range in [x_range, y_range]):
             xfactors, yfactors = self._get_factors(element, ranges)
         framewise = self.framewise
-        streaming = (self.streaming and any(stream._triggering for stream in self.streaming))
+        streaming = (self.streaming and any(stream._triggering and stream.following
+                                            for stream in self.streaming))
         xupdate = ((not (self.model_changed(x_range) or self.model_changed(plot))
                     and (framewise or streaming))
                    or xfactors is not None)

holoviews/streams.py

@@ -473,9 +473,14 @@ class Buffer(Pipe):
     When streaming a DataFrame will reset the DataFrame index by
     default making it available to HoloViews elements as dimensions,
     this may be disabled by setting index=False.
+
+    The ``following`` argument determines whether any plot which is
+    subscribed to this stream will update the axis ranges when an
+    update is pushed. This makes it possible to control whether zooming
+    is allowed while streaming.
     """
 
-    def __init__(self, data, length=1000, index=True, **params):
+    def __init__(self, data, length=1000, index=True, following=True, **params):
         if (util.pd and isinstance(data, util.pd.DataFrame)):
             example = data
         elif isinstance(data, np.ndarray):
@@ -512,6 +517,7 @@ def __init__(self, data, length=1000, index=True, **params):
         params['data'] = example
         super(Buffer, self).__init__(**params)
         self.length = length
+        self.following = following
         self._chunk_length = 0
         self._count = 0
         self._index = index

holoviews/tests/plotting/bokeh/teststreaming.py

@@ -0,0 +1,40 @@
+import numpy as np
+
+from holoviews.core import DynamicMap
+from holoviews.element import Curve 
+from holoviews.streams import Buffer
+
+from .testplot import TestBokehPlot, bokeh_renderer
+
+
+class TestBufferStreamPlot(TestBokehPlot):
+
+    def test_buffer_stream_following(self):
+        stream = Buffer(data={'x': np.array([1]), 'y': np.array([1])}, following=True)
+        dmap = DynamicMap(Curve, streams=[stream])
+
+        plot = bokeh_renderer.get_plot(dmap)
+
+        x_range = plot.handles['x_range']
+        y_range = plot.handles['y_range']
+
+        self.assertEqual(x_range.start, 0)
+        self.assertEqual(x_range.end, 2)
+        self.assertEqual(y_range.start, 0)
+        self.assertEqual(y_range.end, 2)
+
+        stream.send({'x': np.array([2]), 'y': np.array([-1])})
+
+        self.assertEqual(x_range.start, 1)
+        self.assertEqual(x_range.end, 2)
+        self.assertEqual(y_range.start, -1)
+        self.assertEqual(y_range.end, 1)
+
+        stream.following = False
+
+        stream.send({'x': np.array([3]), 'y': np.array([3])})
+
+        self.assertEqual(x_range.start, 1)
+        self.assertEqual(x_range.end, 2)
+        self.assertEqual(y_range.start, -1)
+        self.assertEqual(y_range.end, 1)