Re: change handling of time data for 5.0?

[sfeam <sf...@us...>]

I have made a set of changes in 5.1 CVS related to timefmt.
These changes are not yet in 5.0, but unless they produce unforseen
problems I expect they will be added before the final release of 5.0.

The changes can be summarized briefly.

- The per-axis flag for how to interpret input/output format statements
  has been split into two separate flags
  axis->datatype  continues to select between numeric or time data on input
  axis->tictype (new) specifies how the axis tics are to be handled (output)

- The new flag is set/unset by a corresponding new command option
  set xtics {numeric|time|geographic}

  For now the command "set xdata time" implicitly also does "set xtics time"
  so all old scripts should continue to act as they did previously with 
  regard to time formats

- The per-axis string axis->timefmt has been replaced by a single global
  timefmt string.  This is what the documentation always described,
  although it was not true in the code.
  
  Existing scripts will only see a change if they used the undocumented
  command:   set timefmt y 'something-different-from-x'.
  This case is handled more flexibly in version 5 by 
     plot ... using (timecolumn(N,format1)):(timecolumn(M,format2))

- XYZ positions are printed using time/date formatting if it is active.
  This makes the output consistent with input requirements.
  I.e. the sequence of commands
     set xdata time
     set label "FOO" at "01/02/14,11:23", graph 0.5
     save 'temp'
     load 'temp'
     show label
  Did not act as you would expect in version 4. Now it does.

- The timecolumn(N,format) command tries to detect being called
  with only 1 parameter, as was the case in version 4.  In this
  case the version 5 code now emulates version 4 behavior, so this
  is an improvement, albeit imperfect, in backwards-compatibility.

In short these changes are intended to both extend the flexibility
of time format handling in version 5 and increase the backwards
compatibility with scripts written for version 4.  Please report
any use cases where these goals not succeeded.

	Ethan