[Matplotlib-devel] Units discussion...

Drain, Theodore R (392P) theodore.r.drain at jpl.nasa.gov
Tue Feb 6 17:17:47 EST 2018

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.

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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