Re: [Matplotlib-users] Apparent bug in Data limits with LineCollections

[Christopher B. <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.
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Chr...@no...