[Matplotlib-users] Problem: Multiple plots with same variables plot incorrectly.

[Louis P. <pe...@an...>]

I have some simple code (I've cut it down a lot, but kept the problem) 
that tries to plot two sets of data using the same variables.  It 
appears that re-using the variables from the first plot for the second 
plot affects the first plot.  In fact the two plots become identical.  I 
am guessing that the variables used in the 1st plot are pointed to by 
other matplotlib internal variables (but not plotted until Show() is 
called -- a guess on my part).  In Python fashion when I change a 
variable in preparation for the 2nd plot, the values in the 1st plot 
change, too.

Here is sample code based on some code sent to me by Jeremy O'Donoghue 
as an example of how to embed matplotlib in wxPython.  The problem is in 
the method  plot_data  in the PlotFigure class.  Note how x and y are 
reused for the 2nd plot.  The output should be an 'X' shape, but only 
one line shows up, the 2nd plot line.  If you add new variables to do 
the 2nd plot (e.g. u=zeroes((2,),Float), v=zeroes((2,),Float)) and 
replace the x and y in the 2nd plot with those variables, then it works 
correctly.

Is this good matplotlib behavior?  Is it necessary to use new variables 
for each call of  plot?

I am using wxPython wx-2.6-mac-unicode,  matplotlib 0.82,  Python 2.4, 
on a Macintosh Powerbook G4, OS X 10.4

--------------------------------- The code (self contained as run from 
BBEdit through the Terminal):

#!/usr/local/bin/pythonw

from Numeric import *
import matplotlib
matplotlib.use('WX')
from matplotlib.backends.backend_wx import Toolbar, FigureCanvasWx,\
     FigureManager

from matplotlib.figure import Figure
from matplotlib.axes import Subplot
import matplotlib.numerix as numpy
import wx
   
class PlotFigure(wx.Frame):
    def __init__(self):
        wx.Frame.__init__(self, None, -1, "Test embedded wx.Figure")

        self.fig = Figure((10,8), 75)
        self.canvas = FigureCanvasWx(self, -1, self.fig)
        self.toolbar = Toolbar(self.canvas)
        self.toolbar.Realize()

        # On Windows, default frame size behaviour is incorrect
        # you don't need this under Linux
        tw, th = self.toolbar.GetSizeTuple()
        fw, fh = self.canvas.GetSizeTuple()
        self.toolbar.SetSize(wx.Size(fw, th))

        # Create a figure manager to manage things
        self.figmgr = FigureManager(self.canvas, 1, self)
        # Now put all into a sizer
        sizer = wx.BoxSizer(wx.VERTICAL)
        # This way of adding to sizer allows resizing
        sizer.Add(self.canvas, 1, wx.LEFT|wx.TOP|wx.GROW)
        # Best to allow the toolbar to resize!
        sizer.Add(self.toolbar, 0, wx.GROW)
        self.SetSizer(sizer)
        self.Fit()

    # *** HERE'S THE PROBLEM PART ***
    def plot_data(self):
        # Use ths line if using a toolbar
        a = self.fig.add_subplot(111)
       
        # Or this one if there is no toolbar
        #a = Subplot(self.fig, 111)
        x=zeros((2,),Float)
        y=zeros((2,),Float)
       
        # 1st Plot .................
        x[0]= 1.0
        y[0]= 1.0
        x[1]=-1.0
        y[1]=-1.0
        a.plot(x,y)

        # 2nd Plot .................
        x[0]=-1.0
        y[0]= 1.0
        x[1]= 1.0
        y[1]=-1.0
        a.plot(x,y)
       
        self.toolbar.update()

    def GetToolBar(self):
        # You will need to override GetToolBar if you are using an
        # unmanaged toolbar in your frame
        return self.toolbar
       
if __name__ == '__main__':
    # Plot
    app = wx.PySimpleApp(0)
    frame = PlotFigure()
    frame.plot_data()
    frame.Show()
    app.MainLoop()

-- 
Cheers,

Lou Pecora

Code 6362
Naval Research Lab
Washington, DC  20375
USA
Ph:  +202-767-6002
email:  pe...@an...