[Matplotlib-devel] Suggestion: accept callables as non-xs values arguments

[Thomas Caswell]

Andy,

This is exactly the right place to post things like this.

Fernando Perez has proposed something similar to me in person.  As part of
the label data un-packing discussion there was talk of allowing users to
write expressions in (ex plot('x', 'sqrt(x)', data=df) would plot a square
root function) which is sort of related to this.

What follows is a rather rambling brain dump of me thinking about this ;)

Do you want the functions to be evaluated at Artist creation time or Artist
draw time?

If it is at creation time, see
https://github.com/matplotlib/matplotlib/pull/4829 for how we implemented
something similar for unpacking labeled data.  That might be a reasonable
template for how to do this.  Would tooling to easily bind plotting
functions to the computation be good enough?  For controlling any
additional args/kwargs to the mapping function we can just use `partial`

If it is at _draw_ time, I am a bit more skeptical. But if you can change
the x data, the rest of the data _should_ recompute so that would probably
be a better way to do it long term, but will require much more invasive
changes to the library.

We already have a fair number of mapping functions internally (ex, the
whole transform stack and the data -> norm -> cmap chain), I wonder if
there is a way to generalize any of that?

I am also a bit concerned that this will balloon quickly, for example, for
people plotting histograms, they may want the yerr to be a function of the
y-value (ex Poisson error), not the x-value.  The complexity required to
express everything that would need to be done once we start to go down this
road is a bit daunting.

I have been thinking for a while (like a year) about a massive API
re-working (which I owe the list a long write up on!), I have heard you and
will think about how to integrate this.

One other concern is that our top-level API is _too_ magical rather than
not magical enough.

Tom

On Thu, Sep 17, 2015 at 4:39 PM Andy Buckley <andy.buckley at cern.ch> wrote:

> Hi,
>
> Not sure if this is the best place to post this, but I have an API
> suggestion: as well as iterables, mpl plotting functions could accept
> callable objects to generate y values, errors, etc..
>
> Specifically, I'm thinking that the list of x values would always need
> to be specified as an iterable like now, but that the other arguments
> which currently require an iterable of the same length as the xs could
> alternatively accept callables that are mapped on to the xs list. The
> presence of __iter__ or __call__ attributes could be used to determine
> which approach to use.
>
> I searched a bit and couldn't find any previous mention of this idea,
> and there are callable arguments already used in matplotlib. Thoughts?
> I'd be happy to help with implementation if there is any interest.
>
> Best wishes,
> Andy
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