John,
Thank you for your thorough and thoughtful reply. OK, I am convinced. I
had not realized that the present line-drawing code actually is omitting
nonpositive points, but now I see the Line.get_plottable() method.
I have committed changes to svn that I think will be helpful--maybe good
enough for now. From the CHANGELOG:
2006-12-28 Improved error message for nonpositive input to log
transform; added log kwarg to bar, barh, and hist,
and modified bar method to behave sensibly by default
when the ordinate has a log scale. (This only works
if the log scale is set before or by the call to bar,
hence the utility of the log kwarg.) - EF
Examples:
This makes a sensible plot that behaves well under zooming and panning:
hist(randn(1000), log=True)
show()
The following still generates an exception:
hist(randn(1000))
gca().set_yscale('log')
show()
but the traceback is more informative and ends with:
/usr/local/lib/python2.4/site-packages/matplotlib/patches.py in
draw(self, renderer)
183
184 verts = self.get_verts()
--> 185 tverts = self.get_transform().seq_xy_tups(verts)
186
187 renderer.draw_polygon(gc, rgbFace, tverts)
ValueError: Cannot take log of nonpositive value
I have not put in any form of your suggested addition to set_yscale and
set_xscale. Maybe I should, but I am hoping that the changes above are
sufficient.
Eric
John Hunter wrote:
>>>>>> "Eric" == Eric Firing <[EMAIL PROTECTED]> writes:
>
> Eric> Adjusting zero and negative values (or maybe just zero)
> Eric> would be unacceptable in a numerics library, but in the
> Eric> context of our graphical transforms it is analogous to
> Eric> clipping, and this we do all the time--we don't raise an
> Eric> exception if someone tries to plot outside the box. (This
> Eric> clipping strategy to handle nonpositive values is present
> Eric> already in the LogLocator.)
>
> I'm more comfortable dropping points than I am altering the data.
> Consider some use case like
>
> ax.hist(...)
> rect = Rectangle(...)
> ax.add_patch(rect)
> ax.set_xscale('log')
>
> In set_xscale we only see a bunch of rectangles. User defined
> rectangles may be used to store data (eg the JPL uses custom bars with
> xlimits that are the start and stop times when orbiting spacecraft are
> within view of a ground station). Some of these users will also want
> to "pick" the rectangle and inspect the data values. If we are
> altering them, they have no guarantee that what they put in is what
> they get out. Now we might argue that they get what they deserve if
> they are using invalid data for a log scale, but one good solution in
> my view is to fail noisily with helpful messages, and provide an easy
> interface to do it right.
>
> But if I am missing your point or you have an implementation that will
> address these concerns, I'm certainly open to them. One possibility
> is to flag artists that we create internally (eg hist) and take more
> liberty with these, or have some flag like the Artist "clip_on"
> property which allows the user to control whether their data is
> mutable to support "helpful" alterations for log or other
> transformations.
>
> Eric> We can use such a small adjustment value that a problem such
> Eric> as you mention above is highly unlikely--and note that
> Eric> floating point itself has limitations, and does not permit
> Eric> arbitrarily small or large numbers. Furthermore, note that
>
> Eric> the user can always take advantage of the bottom kwarg. And
> Eric> if in some extreme case the user has not used the bottom
> Eric> kwarg and the bars really are shorter than the adjustment
> Eric> value, it will probably be quite obvious.
>
> The other thing to think about is choosing a bottom so that the
> natural range of the tops is revealed. Eg if the bottom is 1e-200,
> all the bars will appear the same height in many cases.
>
> Eric> It is in ordinary line plotting that adjusting the value
> Eric> could be misleading--it plots an extremely small number (if
> Eric> the data limits are set to include it) instead of zero.
> Eric> Maybe this is enough of a drawback to nix the whole idea.
>
> I am happy with the current behavior of culling the non-positive
> points. matlab does it and noone has complained here. There is a
> difference in lines created with "plot" and bars created with hist: in
> the former case the users explicitly picked the x,y points. In the
> latter they implicitly do so with a default bottom kwarg that they may
> have overlooked. This suggests to me that we need not treat the two
> cases the same.
>
> Eric> Every alternative that you propose is more complicated and
> Eric> less comprehensive than the low-level adjustment, however,
> Eric> and I see little if any real advantage to the alternatives.
>
> If you would like to take a stab at an implementation I am happy to be
> persuaded (with caveats below). In the simple case of
>
> ax.hist()
> ax.set_xscale('log')
>
> this would indeed be fairly easy because you know how the data were
> created. In the general case where the user has added lots-o-patches
> to the axes, it may not be easy to do well. I'm still inclined to the
> "explicit is better than implicit" and either require them to do one of
>
> 1) use the bottom kwarg
>
> 2) set log scaling before calling hist -- we can make the default
> bottom=None and do different things for linear and log scaling
>
> 3) use a loghist function
>
> Eric> If you still don't want the adjustment, then the easiest way
> Eric> to improve the error message would be to raise a Python
> Eric> exception instead of a c++ error in places like
>
> Eric> for(int i=0; i < length; i++) { if (x<=0) { throw
> Eric> std::domain_error("Cannot take log of nonpositive value"); }
> Eric> else newx[i] = log10(x[i]);
> Eric> }
>
> Eric> The domain error message above is informative, but it never
> Eric> makes it out to the user.
>
> I really don't feel too strongly about this -- my gut reaction is that
> a helpful message and an easy way to fix it is enough and it won't get
> us into a possible quagmire of trying to be too smart. Personally, I
> don't like it when computers try to be too helpful (think MS windows
> and clippy); I like it when they do what I tell them to do. With the
> snippet I posted previously we can easily warn them before doing the
> transformation and with bottom=None we can handle the case when the
> log scale is set before the call to hist. That in conjunction with
> some additional docstrings in hist should work reasonably well.
>
> That said, if you want to try something more ambitious I won't get in
> your way. I recommend at a minimum that you have some artist flag
> that governs whether mpl can make helpful data alterations (just as we
> do with clip) so the power user can turn it off.
>
> JDH
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