Re: [matplotlib-devel] Stacked plotting in matplotlib

[Eric F. <ef...@ha...>]

On 2012/08/23 11:55 AM, Damon McDougall wrote:
> Hey Nic,
>
> Thanks for bringing this up. I was the author for #819, so I'd like to
> get some dicussion going on this, too. Sorry for the delay, I was in the
> midst of writing a thesis, which I am now free of.
>
> On Sun, Aug 12, 2012 at 11:51:24PM -0500, Nic Eggert wrote:
>> Hi all,
>>
>> I'd like to bring up a question spurred by PRs #847(mine) and #819
>> (recently accepted). These PRs both deal with stacked plots. #819 adds the
>> stackplot function to axes.py as a new function, which plots different 2-d
>> datasets stacked atop each other. #847 slightly modifies the functioning of
>> `hist` in axes.py by adding a new kwarg which allows datasets to be
>> stacked. Currently this is only possible using the `barstacked` histtype.
>> #847 makes it also work with the `step` and `stepfilled` histtypes.
>>
>> One of the issues that has been raised in the comments of #847 is whether
>> we want to take this opportunity to come up with a unified way to handle
>> "stacked-ness". Michael Droettboom suggested I raise this issue on this
>> list. So far, there are 3 different approaches:
>>
>> 1. The state before #819. AFAIK the only way to do any sort of stacking was
>> to call `hist` with `histtype="barstacked"`. This treats stacked histograms
>> as a different type of histogram than non-stacked histograms. One of my
>> motivations for writing #847 was to get stacked step and stepfilled
>> histograms, which would require adding several new histtypes (stepstacked
>> and stepfilledstacked). It seems to me that histtype mostly controls the
>> style of the histogram plotted, and shouldn't have anything to do with
>> "stacked-ness", so I think this is kind of clunky.
>>
>> 2. The approach I take in #847. Add a new kwarg which controls whether or
>> not multiple datasets are stacked. I think this is the cleanest
>> implementation, although that's probably obvious because it's how I wrote
>> my PR. To keep everything consistent in this approach, we should remove the
>> stackplot function added in #819, and move that functionality to the `plot`
>> function, adding a `stacked` kwarg there.
>>
>> 3. The approach of #819. With this approach, we would add a separate
>> function to handle stacked versions of different plots. I'd re-write #847
>> as a new function called `stackhist`. This approach, IMO, doesn't scale
>> well if we want to add "stacked-ness" to more plot types in the future.
>
> I'm in favour of numero dos, even though for #819 I took approach number
> 3. I didn't really think about the bigger picture here with regards to
> stackedness of other plot types. But since seeing your stacked histogram
> changeset, this seems like a more sensible route.
>
> I say this with zero authority, though.
>
> It'd be nice to have a few people chime in with their two cents.

OK, here are mine: I oppose overloading plot with a "stacked" kwarg and 
functionality.  It is complicated enough as it is.  I don't see any 
problem with having "stackplot" and hist(..., stacked=True).  They are 
just not all that similar.  Nor are "plot" and "stackplot" so very 
similar.  But stacked and non-stacked histograms *are* very similar, so 
using the kwarg to turn on stacking there makes sense.

Elaborating slightly: stacking in plot makes sense only when there is a 
single abcissa in the data set, but plot supports inputs for which this 
is not the case; that means that using a stacked kwarg would require 
explaining this, and trapping invalid inputs when stacked is True. 
Messy.  Much neater to have a separate function.

In the case of a histogram, there is a single set of bins, so a single 
abcissa.  Therefore turning on stacking only affects the way the lines 
are displayed, and does not require additional input validity checking.


I would be cautious about looking around for more places to add a 
"stacked" kwarg.  Where is it really needed?  Let's try to keep mpl from 
getting more complicated than necessary.

Eric

>
>> Please take a look at this and send comments about these proposals or any
>> others you might have. I hope the community can come to a consensus which
>> unifies the handling of stacked-ness.
>>
>> Whatever we end up choosing, I think adding a stacked step histogram will
>> make it much easier to promote the use of mpl in high energy physics, where
>> we use this style of plot frequently.
>>
>> Thanks,
>>
>> Nic Eggert
>> Graduate Fellow
>> Cornell University
>
>