Added scrolling plot examples

examples/__main__.py

@@ -31,6 +31,7 @@ examples = OrderedDict([
     ('Histograms', 'histogram.py'),
     ('Auto-range', 'PlotAutoRange.py'),
     ('Remote Plotting', 'RemoteSpeedTest.py'),
+    ('Scrolling plots', 'scrollingPlots.py'),
     ('HDF5 big data', 'hdf5.py'),
     ('Demos', OrderedDict([
         ('Optics', 'optics_demos.py'),

examples/scrollingPlots.py

@@ -0,0 +1,118 @@
+# -*- coding: utf-8 -*-
+"""
+Various methods of drawing scrolling plots.
+"""
+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
+
+win = pg.GraphicsWindow()
+win.setWindowTitle('pyqtgraph example: Scrolling Plots')
+
+
+# 1) Simplest approach -- update data in the array such that plot appears to scroll
+#    In these examples, the array size is fixed.
+p1 = win.addPlot()
+p2 = win.addPlot()
+data1 = np.random.normal(size=300)
+curve1 = p1.plot(data1)
+curve2 = p2.plot(data1)
+ptr1 = 0
+def update1():
+    global data1, curve1, ptr1
+    data1[:-1] = data1[1:]  # shift data in the array one sample left
+                            # (see also: np.roll)
+    data1[-1] = np.random.normal()
+    curve1.setData(data1)
+    
+    ptr1 += 1
+    curve2.setData(data1)
+    curve2.setPos(ptr1, 0)
+    
+
+# 2) Allow data to accumulate. In these examples, the array doubles in length
+#    whenever it is full. 
+win.nextRow()
+p3 = win.addPlot()
+p4 = win.addPlot()
+# Use automatic downsampling and clipping to reduce the drawing load
+p3.setDownsampling(mode='peak')
+p4.setDownsampling(mode='peak')
+p3.setClipToView(True)
+p4.setClipToView(True)
+p3.setRange(xRange=[-100, 0])
+p3.setLimits(xMax=0)
+curve3 = p3.plot()
+curve4 = p4.plot()
+
+data3 = np.empty(100)
+ptr3 = 0
+
+def update2():
+    global data3, ptr3
+    data3[ptr3] = np.random.normal()
+    ptr3 += 1
+    if ptr3 >= data3.shape[0]:
+        tmp = data3
+        data3 = np.empty(data3.shape[0] * 2)
+        data3[:tmp.shape[0]] = tmp
+    curve3.setData(data3[:ptr3])
+    curve3.setPos(-ptr3, 0)
+    curve4.setData(data3[:ptr3])
+
+
+# 3) Plot in chunks, adding one new plot curve for every 100 samples
+chunkSize = 100
+# Remove chunks after we have 10
+maxChunks = 10
+startTime = pg.ptime.time()
+win.nextRow()
+p5 = win.addPlot(colspan=2)
+p5.setLabel('bottom', 'Time', 's')
+p5.setXRange(-10, 0)
+curves = []
+data5 = np.empty((chunkSize+1,2))
+ptr5 = 0
+
+def update3():
+    global p5, data5, ptr5, curves
+    now = pg.ptime.time()
+    for c in curves:
+        c.setPos(-(now-startTime), 0)
+    
+    i = ptr5 % chunkSize
+    if i == 0:
+        curve = p5.plot()
+        curves.append(curve)
+        last = data5[-1]
+        data5 = np.empty((chunkSize+1,2))        
+        data5[0] = last
+        while len(curves) > maxChunks:
+            c = curves.pop(0)
+            p5.removeItem(c)
+    else:
+        curve = curves[-1]
+    data5[i+1,0] = now - startTime
+    data5[i+1,1] = np.random.normal()
+    curve.setData(x=data5[:i+2, 0], y=data5[:i+2, 1])
+    ptr5 += 1
+
+
+# update all plots
+def update():
+    update1()
+    update2()
+    update3()
+timer = pg.QtCore.QTimer()
+timer.timeout.connect(update)
+timer.start(50)
+
+
+
+## Start Qt event loop unless running in interactive mode or using pyside.
+if __name__ == '__main__':
+    import sys
+    if (sys.flags.interactive != 1) or not hasattr(QtCore, 'PYQT_VERSION'):
+        QtGui.QApplication.instance().exec_()

pyqtgraph/graphicsItems/PlotDataItem.py

@@ -546,7 +546,7 @@ class PlotDataItem(GraphicsObject):
                 if view is None or not view.autoRangeEnabled()[0]:
                     # this option presumes that x-values have uniform spacing
                     range = self.viewRect()
-                    if range is not None:
+                    if range is not None and len(x) > 1:
                         dx = float(x[-1]-x[0]) / (len(x)-1)
                         # clip to visible region extended by downsampling value
                         x0 = np.clip(int((range.left()-x[0])/dx)-1*ds , 0, len(x)-1)