python - Matplotlib: scrolling plot

Question

I'm new to Python and I want to implement a scrolling plot for a very long time series data. I've found an example from Matplotlib as follows.

http://scipy-cookbook.readthedocs.io/items/Matplotlib_ScrollingPlot.html

When I run the example from the link, I found every time I scroll the plot and release the scrollbar, the scrollbar returns to the beginning. Want to scroll to the next position? I need to start to scroll from the beginning again.

I want to understand why it happens and how to fix it.

Answer 1

Here's an improved version of the example. (Disclaimer: I started digging into it half an hour ago, never before used wx/matplotlib scrollbars so there might be a much better solution.)

The path I took: first I checked the wx scroll events, then found out that the canvas is FigureCanvasWxAgg derived from wxPanel, inheriting wxWindow methods. There you may find the scroll position handling methods GetScrollPos and SetScrollPos.

from numpy import arange, sin, pi, float, size

import matplotlib
matplotlib.use('WXAgg')
from matplotlib.backends.backend_wxagg import FigureCanvasWxAgg
from matplotlib.figure import Figure

import wx

class MyFrame(wx.Frame):
    def __init__(self, parent, id):
        wx.Frame.__init__(self,parent, id, 'scrollable plot',
                style=wx.DEFAULT_FRAME_STYLE ^ wx.RESIZE_BORDER,
                size=(800, 400))
        self.panel = wx.Panel(self, -1)

        self.fig = Figure((5, 4), 75)
        self.canvas = FigureCanvasWxAgg(self.panel, -1, self.fig)
        self.scroll_range = 400
        self.canvas.SetScrollbar(wx.HORIZONTAL, 0, 5,
                                 self.scroll_range)

        sizer = wx.BoxSizer(wx.VERTICAL)
        sizer.Add(self.canvas, -1, wx.EXPAND)

        self.panel.SetSizer(sizer)
        self.panel.Fit()

        self.init_data()
        self.init_plot()

        self.canvas.Bind(wx.EVT_SCROLLWIN, self.OnScrollEvt)

    def init_data(self):

        # Generate some data to plot:
        self.dt = 0.01
        self.t = arange(0,5,self.dt)
        self.x = sin(2*pi*self.t)

        # Extents of data sequence:
        self.i_min = 0
        self.i_max = len(self.t)

        # Size of plot window:
        self.i_window = 100

        # Indices of data interval to be plotted:
        self.i_start = 0
        self.i_end = self.i_start + self.i_window

    def init_plot(self):
        self.axes = self.fig.add_subplot(111)
        self.plot_data = 
                  self.axes.plot(self.t[self.i_start:self.i_end],
                                 self.x[self.i_start:self.i_end])[0]

    def draw_plot(self):

        # Update data in plot:
        self.plot_data.set_xdata(self.t[self.i_start:self.i_end])
        self.plot_data.set_ydata(self.x[self.i_start:self.i_end])

        # Adjust plot limits:
        self.axes.set_xlim((min(self.t[self.i_start:self.i_end]),
                           max(self.t[self.i_start:self.i_end])))
        self.axes.set_ylim((min(self.x[self.i_start:self.i_end]),
                            max(self.x[self.i_start:self.i_end])))

        # Redraw:
        self.canvas.draw()

    def update_scrollpos(self, new_pos):
        self.i_start = self.i_min + new_pos
        self.i_end = self.i_min + self.i_window + new_pos
        self.canvas.SetScrollPos(wx.HORIZONTAL, new_pos)
        self.draw_plot()

    def OnScrollEvt(self, event):
        evtype = event.GetEventType()

        if evtype == wx.EVT_SCROLLWIN_THUMBTRACK.typeId:
            pos = event.GetPosition()
            self.update_scrollpos(pos)
        elif evtype == wx.EVT_SCROLLWIN_LINEDOWN.typeId:
            pos = self.canvas.GetScrollPos(wx.HORIZONTAL)
            self.update_scrollpos(pos + 1)
        elif evtype == wx.EVT_SCROLLWIN_LINEUP.typeId:
            pos = self.canvas.GetScrollPos(wx.HORIZONTAL)
            self.update_scrollpos(pos - 1)
        elif evtype == wx.EVT_SCROLLWIN_PAGEUP.typeId:
            pos = self.canvas.GetScrollPos(wx.HORIZONTAL)
            self.update_scrollpos(pos - 10)
        elif evtype == wx.EVT_SCROLLWIN_PAGEDOWN.typeId:
            pos = self.canvas.GetScrollPos(wx.HORIZONTAL)
            self.update_scrollpos(pos + 10)
        else:
            print "unhandled scroll event, type id:", evtype

class MyApp(wx.App):
    def OnInit(self):
        self.frame = MyFrame(parent=None,id=-1)
        self.frame.Show()
        self.SetTopWindow(self.frame)
        return True

if __name__ == '__main__':
    app = MyApp()
    app.MainLoop()

You may adjust e.g. the increments for PAGEUP/PAGEDOWN if you feel it too slow.

Also if you wish, the events can be handled separately setting up the specific event handlers instead of their collection EVT_SCROLLWIN, then instead of if/elifs there will be OnScrollPageUpEvt etc.