[Matplotlib-devel] Units discussion...

[Jody Klymak]

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