matplotlib-users Mailing List for matplotlib (Page 15)

Christopher Barker wrote:
> Christopher Barker wrote:
> 
>> Maybe I'll try to make a small sample that demonstrates the issue. 
>> stay tuned.
> 
> 
> OK, read the previous posts for background, or ignore them, and just 
> read this one. I made a small sample, and found that using a 
> LineCollection with axes.add_collection does something weird to the data 
> limits that axes sets. I've enclosed a small sample that demonstrates 
> the problem.


Chris,

Attached is a slight modification of your test case, with annotations. 
I think it does what you want--sort of.  But I agree that there is a 
bug.  After chasing references backwards and forwards through axes.py, 
axis.py, ticker.py, and _transforms.cpp, I don't quite understand how 
everything works, but I think this is part of the problem:

axes.has_data() is being used to determine whether to update the dataLim 
from scratch, or to start with previous values.  axes.has_data() is True 
if any drawing artist (Line, Collection, etc.) has been added to the 
axes.  So, after clearing the axes, adding a LineCollection tells the 
dataLim update method (called by the subsequent plot()) to use the old 
information, even though Collections (unlike Lines) do not change the 
dataLim.  In other words, in your original script, I think that adding 
the collection was preventing plot() from setting the axes to a smaller 
value.

Workarounds: either set the dataLim explicitly, or add the collection 
after the plot command (as in the attached modified script).  The latter 
only works if the plot command sets the dataLim to be large enough to 
cover everything in the collection.

I would like to make a genuine bugfix, but I do not yet understand all 
this well enough to do so right now.  Maybe John will chime in with a 
good solution.

Eric

Just for the record, I've solved all this, for the moment, by clearing 
the entire figure, and starting again with a brand new axes. It does 
seem like there's a bug in the axes.clear() method, however, I should be 
able to just .clear() and add stuff again, shouldn't I?

-Chris

-- 
Christopher Barker, Ph.D.
Oceanographer
                                     		
NOAA/OR&R/HAZMAT         (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

Chr...@no...

Christopher Barker wrote:

> Maybe I'll try to make a small sample that demonstrates the issue. stay 
> tuned.

OK, read the previous posts for background, or ignore them, and just 
read this one. I made a small sample, and found that using a 
LineCollection with axes.add_collection does something weird to the data 
limits that axes sets. I've enclosed a small sample that demonstrates 
the problem.

Here is the plotting code itself:

# Do some plotting
ax.plot(offsets[:,0], offsets[:,1], 'o')

# clear the axes, and try again, with different data.
ax.clear()
offsets *= .1
coll = LineCollection(segments, offsets=offsets,
                       transOffset=ax.transData, # transforms the x,y 
offsets
                       )
# points/72.*dpi = pixels -- see matplotlib.transforms
trans = scale_transform(fig.dpi/Value(72.), fig.dpi/Value(72.))
coll.set_transform(trans)  # the points to pixels transform

ax.add_collection(coll)
ax.plot(offsets[:,0], offsets[:,1], 'o')
ax.autoscale_view()

pylab.show()

running this, you get the axes set to data limits appropriate to the 
first ax.plot call, before the ax.clear() call. (the second has values 
1/10 as large.

However, if you don't do the first plot call, it all scales correctly. 
Now it's even weirder. If you do the first plot call (with the larger 
data), but comment out add_collection() call, it scales correctly. So it 
seems that using add_collection someone fixes the data limits to the 
previous value, even after the ax.clear() call. AFAICT, looking at the 
axes class code, this shouldn't happen. add_collection doesn't do much 
at all.

Also, after axes.clear() is called, ax.has_data() returns False, so when
axes.update_datalim is called, the old dataLim should be ignored.

by the way, it looks like axes.cla() should perhaps have this in it:

self.dataLim = unit_bbox()

or maybe:

self._set_lim_and_transforms()


I've gotten a bit lost in all this: HELP!

-Chris

-- 
Christopher Barker, Ph.D.
Oceanographer
                                     		
NOAA/OR&R/HAZMAT         (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

Chr...@no...