I have large time-traces that must be inspected visually, so I need a fast scrolling tool.
How can I achieve the fastest Maplotlib/Pyside scrolling?
Right know, I added a PySide scroll-bar to a MPL figure and update the x-range of the plot with set_xlim() method. This is not fast enough especially because in the final application I have at least 8 time-traces in different subplots that must all scroll together. A figure of the plot is attached.
Is there room for improvement?
Here I attach the demo code that demonstrate the relatively low scrolling. It's long but it's almost all boiler-plate code. The interesting bit (that needs improvement) is in xpos_changed() method where the plot xlimits are changed.
EDIT: Below I incorporated some micro-optimizations suggested by tcaswell, but the update speed is not improved.
from PySide import QtGui, QtCore import pylab as plt import numpy as np N_SAMPLES = 1e6 def test_plot(): time = np.arange(N_SAMPLES)*1e-3 sample = np.random.randn(N_SAMPLES) plt.plot(time, sample, label="Gaussian noise") plt.title("1000s Timetrace \n (use the slider to scroll and the spin-box to set the width)") plt.xlabel('Time (s)') plt.legend(fancybox=True) q = ScrollingToolQT(plt.gcf(), scroll_step=10) return q # WARNING: it's important to return this object otherwise # python will delete the reference and the GUI will not respond! class ScrollingToolQT(object): def __init__(self, fig, scroll_step=10): # Setup data range variables for scrolling self.fig = fig self.scroll_step = scroll_step self.xmin, self.xmax = fig.axes[0].get_xlim() self.width = 1 # axis units self.pos = 0 # axis units self.scale = 1e3 # conversion betweeen scrolling units and axis units # Save some MPL shortcuts self.ax = self.fig.axes[0] self.draw = self.fig.canvas.draw #self.draw_idle = self.fig.canvas.draw_idle # Retrive the QMainWindow used by current figure and add a toolbar # to host the new widgets QMainWin = fig.canvas.parent() toolbar = QtGui.QToolBar(QMainWin) QMainWin.addToolBar(QtCore.Qt.BottomToolBarArea, toolbar) # Create the slider and spinbox for x-axis scrolling in toolbar self.set_slider(toolbar) self.set_spinbox(toolbar) # Set the initial xlimits coherently with values in slider and spinbox self.ax.set_xlim(self.pos,self.pos+self.width) self.draw() def set_slider(self, parent): self.slider = QtGui.QSlider(QtCore.Qt.Horizontal, parent=parent) self.slider.setTickPosition(QtGui.QSlider.TicksAbove) self.slider.setTickInterval((self.xmax-self.xmin)/10.*self.scale) self.slider.setMinimum(self.xmin*self.scale) self.slider.setMaximum((self.xmax-self.width)*self.scale) self.slider.setSingleStep(self.width*self.scale/4.) self.slider.setPageStep(self.scroll_step*self.width*self.scale) self.slider.setValue(self.pos*self.scale) # set the initial position self.slider.valueChanged.connect(self.xpos_changed) parent.addWidget(self.slider) def set_spinbox(self, parent): self.spinb = QtGui.QDoubleSpinBox(parent=parent) self.spinb.setDecimals(3) self.spinb.setRange(0.001,3600.) self.spinb.setSuffix(" s") self.spinb.setValue(self.width) # set the initial width self.spinb.valueChanged.connect(self.xwidth_changed) parent.addWidget(self.spinb) def xpos_changed(self, pos): #pprint("Position (in scroll units) %f\n" %pos) pos /= self.scale self.ax.set_xlim(pos, pos+self.width) self.draw() def xwidth_changed(self, width): #pprint("Width (axis units) %f\n" % step) if width <= 0: return self.width = width self.slider.setSingleStep(self.width*self.scale/5.) self.slider.setPageStep(self.scroll_step*self.width*self.scale) old_xlim = self.ax.get_xlim() self.xpos_changed(old_xlim[0]*self.scale) if __name__ == "__main__": q = test_plot() plt.show() 
