[Matplotlib-devel] Units discussion...

David Stansby dstansby at gmail.com
Wed Feb 7 06:42:49 EST 2018

Practically, I think what we are proposing is that for unit support the
user must supply two functions for each axis:

   - A mapping from your unit objects to floating point numbers
   - A mapping from those floats back to your unit objects

As far as I know function 2 is new, and doesn't need to be supplied at the
moment. Doing this would mean we can convert units as soon as they enter
Matplotlib, only ever have to deal with floating point numbers internally,
and then use the second function as late as possible when the user requests
stuff like e.g. the axis limits.

Also worth noting that any major change like this will go in to Matplotlib
3.0 at the earliest, so will be python 3 only.


On 7 February 2018 at 06:06, Jody Klymak <jklymak at uvic.ca> wrote:

> Dear Ted,
> Thanks so much for engaging on this.
> Don’t worry, nothing at all is changing w/o substantial back and forth,
> and OK from downstream users.   I actually don’t think it’ll be a huge
> change, probably just some clean up and better documentation.
> FWIW, I’ve not personally done much programming w/ units, just been a bit
> perplexed by their inconsistent and (to my simple mind) convoluted
> application in the codebase.  Having experience from people who try to use
> them everyday will be absolutely key.
> Cheers,   Jody
> > On Feb 6, 2018, at  14:17 PM, Drain, Theodore R (392P) <
> theodore.r.drain at jpl.nasa.gov> wrote:
> >
> > We use units for everything in our system (in fact, we funded John
> Hunter originally to add in a unit system so we could use MPL) so it's a
> crucial system for us.  In our system, we have our own time classes (which
> handle relativistic time frames as well as much higher precision
> representations) and a custom unit system for floating point values.
> >
> > I think it's important to talk about these changes in concrete terms.  I
> understand the words you're using,  but I'm not really clear on what the
> real proposed changes are.  For example, the current unit API returns a
> units.AxisInfo object so the converter can set the formatter and locators
> to use.  Is that what you mean in the 2nd paragraph about ticks and
> labels?  Or is that changing?
> >
> > The current unit api is pretty simple and in units.ConversionInterface.
> Are any of these changes going to change the conversion API?  (note - I'm
> not against changing it - I'm just not sure if there are any changes or
> not).
> >
> > Another thing to consider:  many of the examples people use are scripts
> which make a plot and stop.  But there are other use cases which are more
> complicated and stress the system in different ways.  We write several GUI
> applications (in PyQt) that use MPL for plotting.  In these cases, the user
> is interacting with the plot to add and remove artists, change styles,
> modify data, etc etc.  So having a good object oriented API for modifying
> things after construction is important for this to work.  So when units are
> involved, it can't be a "convert once at construction" and never touch
> units again.   We are constantly adjusting limits, moving artists, etc in
> unitized space after the plot is created.
> >
> > So in addition to the ConversionInterface API, I think there are other
> items that would be useful to explicitly spelled out.  Things like which
> API's in MPL should accept units and which won't and which methods return
> unitized data and which don't.   It would be nice if there was a clear
> policy on this.  Maybe one exists and I'm not aware of it - it would be
> helpful to repeat it in a discussion on changing the unit system.
> Obviously I would love to have every method accept and return unitized data
> :-).
> >
> > I bring this up because I was just working on a hover/annotation class
> that needed to move a single annotation artist with the mouse.  To move the
> annotation box the way I needed to, I had to set to one private member
> variable, call two set methods, use attribute assignment for one value, and
> set one semi-public member variable - some of which work with units and
> some of which didn't.  I think having a clear "this kind of method
> accepts/returns units" policy would help when people are adding new
> accessors/methods/variables to make it more clear what kind of data is
> acceptable in each.
> >
> > Ted
> > ps: I may be able to help with some resources to work on any unit
> upgrades, but to make that happen I need to get a clear statement of what
> problem is being solved and the scope of the work so I can explain to our
> management why it's important.
> >
> > ________________________________________
> > From: Matplotlib-devel <matplotlib-devel-bounces+ted.drain=
> jpl.nasa.gov at python.org> on behalf of Jody Klymak <jklymak at uvic.ca>
> > Sent: Saturday, February 3, 2018 9:25 PM
> > To: matplotlib development list
> > Subject: [Matplotlib-devel] Units discussion...
> >
> > Hi all,
> >
> > To carry on the gitter discussion about unit handling, hopefully to lead
> to a more stringent documentation and implimentation….
> >
> > In response to @anntzer I thought about the units support a bit - it
> seems that rather than a transform, a more straightforward approach is to
> have the converter map to float arrays in a unique way.  This float mapping
> would be completely analogous to `date2num` in `dates`, in that it doesn’t
> change and is perfectly invertible without matplotlib ever knowing about
> the unit information, though the axis could store it for the the tick
> locators and formatters.  It would also have an inverse that would supply
> data back to the user in unit-aware data (though not necessarily in the
> unit that the user supplied.  e.g. if they supply 8*in, the and the
> converter converts everything to meter floats, then the returned unitized
> inverse would be 0.203*m, or whatever convention the converter wants to
> supply.).
> >
> > User “unit” control, i.e. making the plot in inches instead of m, would
> be accomplished with ticks locators and formatters.  Matplotlib would never
> directly convert between cm and inches (any more than it converts from days
> to hours for dates), the downstream-supplied tick formatter and labeller
> would do it.
> >
> > Each axis would only get one converter, set by the first call to the
> axis. Subsequent calls to the axis would pass all data (including bare
> floats) to the converter.  If the converter wants to pass bare floats then
> it can do so.  If it wants to accept other data types then it can do so.
> It should be possible for the user to clear or set the converter, but then
> they should know what they are doing and why.
> >
> > Whats missing?  I don’t think this is wildly different than what we
> have, but maybe a bit more clear.
> >
> > Cheers,   Jody
> >
> >
> >
> >
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