[Matplotlib-users] problems with semilogy and debian

[Eugen W. <eug...@jk...>]

Hi there
 I use matplotlib from within ipyton (simply by calling $ipython -pylab
in a shell window). I'm using the debian packages as mentioned on the
matplotlib homepage on a Debian sarge system. 
Everything works fine except the semilogy command behaves a bit strange
(I'm new to matplotlib so it is maybe my mistake). 
After starting ipython in pylab mode I do the following:

In [1]: x=arrayrange(0.0,100.0);

In [2]: plot(x)
Out[2]: [<matplotlib.lines.Line2D instance at 0x4437a30c>]

until here everything works fine (I use the TkAgg interface for
interactive plotting, hold is set to False). 
If I continue now with

In [3]: semilogy(x)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call
last)

/home/eugen/<console>

/usr/lib/python2.3/site-packages/matplotlib/pylab.py in semilogy(*args,
**kwargs)
   2089         hold(b)
   2090     else:
-> 2091         draw_if_interactive()
   2092
   2093         hold(b)

/usr/lib/python2.3/site-packages/matplotlib/backends/__init__.py in
draw_if_interactive()
     39     def draw_if_interactive():
     40         draw_if_interactive._called = True
---> 41         __draw_int()
     42     # Flag to store state, so external callers (like ipython)
can keep track
     43     # of draw calls.

/usr/lib/python2.3/site-packages/matplotlib/backends/backend_tkagg.py in
draw_if_interactive()
     56         figManager =  Gcf.get_active()
     57         if figManager is not None:
---> 58             figManager.show()
     59
     60

/usr/lib/python2.3/site-packages/matplotlib/backends/backend_tkagg.py in
show(self)
    275             # anim.py requires this
    276             if sys.platform=='win32' : self.window.update()
--> 277         else: self.canvas.draw()
    278         self._shown = True
    279

/usr/lib/python2.3/site-packages/matplotlib/backends/backend_tkagg.py in
draw(self)
    140
    141     def draw(self):
--> 142         FigureCanvasAgg.draw(self)
    143         tkagg.blit(self._tkphoto, self.renderer._renderer, 2)
    144         self._master.update_idletasks()

/usr/lib/python2.3/site-packages/matplotlib/backends/backend_agg.py in
draw(self)
    306             self.renderer = RendererAgg(w, h, self.figure.dpi)
    307             self._lastKey = key
--> 308         self.figure.draw(self.renderer)
    309
    310     def tostring_rgb(self):

/usr/lib/python2.3/site-packages/matplotlib/figure.py in draw(self,
renderer)
    332
    333         # render the axes
--> 334         for a in self.axes: a.draw(renderer)
    335
    336         # render the figure text

/usr/lib/python2.3/site-packages/matplotlib/axes.py in draw(self,
renderer)
   1167         if not self.get_visible(): return
   1168         renderer.open_group('axes')
-> 1169         self.transData.freeze()  # eval the lazy objects
   1170         self.transAxes.freeze()  # eval the lazy objects
   1171         if self.axison:

ValueError: Cannot take log of nonpositive value

you see what happens. Since the array starts with 0.0 the error message
is ok. But 

In [4]: x=x+1.0;

In [5]: semilogy(x)

gives the same error. 
Also a subsequent clf or cla command could not solve the problem. 
Has anyone of you an idea what is going on here (or what I'm doing
wrong)? My matplotlib version is 0.71-1.

best regards
  Eugen Wintersberger

-- 
Eugen Wintersberger <eug...@jk...>