Re: irreversibly reversing range

[Ethan M. <merritt@u.washington.edu>]

On Saturday, 08 September 2012, Daniel J Sebald wrote:
> I see there is new behavior for the option "reverse" for gnuplot version 
> 4.7+.  It seems to me the change is that "{no}reverse" now only applies 
> to autoscaling.  (In fact, the documentation ostensibly says that.)
> 
> Could someone write a one or two sentence summary of why the change. 
> E.g., the previous behavior was too confusing or caused a conflict?  I 
> just want to be able to explain why the change if asked.

The old behaviour was bizarre.  Example:
gnuplot> set xrange [1:0]
gnuplot> show xrange
        set xrange [ 0.00000 : 1.00000 ] reverse nowriteback
gnuplot> set xrange [0:1]
gnuplot> show xrange
        set xrange [ 0.00000 : 1.00000 ] reverse nowriteback

Notice that the "reverse" property is now stuck, and no ordinary
sequence of autoscaling/autorange/set xrange commands will unstick it.  
Only an explicit "set xr [<something>:<something>] noreverse"
will work, but inside a script you can't know in advance when this
might be necessary, and at that point you may not know what the
correct <something> is anyhow.

See also various old bug reports going back 10 years or so, e.g. #230686.

> 
> Reverse axes aren't used real often, except in the case of y-axis and 
> images where it is quite common to see the origin in the upper left 
> corner of a plot.  (Octave will need a change to address this.)

You have neglected that case of inverted axis scaling   x2 = F(1/x1)
Consider, for example:
   http://skuld.bmsc.washington.edu/people/merritt/gnuplot/linkedaxes/

> I also want to confirm that the new behavior is really what is desired. 
>   If there is something that may have been wrong with the previous 
> behavior it is that the old syntax was too flexible.  I think it was 
> Ethan that pointed out controlling the axis from within the plot command 
> itself was problematic, e.g.,
> 
> plot [0:500] x

Yeah, that too.   If the "reverse" flag got stuck on, the in-line
range specifiers didn't work reliably.

	Ethan

 
> I see the point in the sense that multiple items with different range 
> settings can be a conflict.  Maybe we shouldn't think of the range 
> specified within the plot command as "axis range".
> 
> Perhaps it is the fact that range setting itself can control the 
> orientation of an axes, i.e., [500:0].  If that were disallowed, then 
> there would still be an ability to control range within the plot 
> command, but not be able to control the orientation of the axis.
> 
> So rather than the change "reverse only applies to autoscaling", another 
> change could have been "range limits no longer control orientation".  So 
> that would mean
> 
> set xrange [0:500]
> set xrange [500:0]
> plot [0:500]
> plot [500:0]
> 
> all mean the same thing, i.e., the xaxis range is from 0 to 500 using 
> the typical Cartesian orientation.  Also
> 
> set xrange [0:500] reverse
> set xrange [500:0] reverse
> set xrange reverse
> plot [0:500]
> set xrange reverse
> plot [500:0]
> 
> would all mean the same thing, i.e., the xaxis range is from 0 to 500 
> but using the reverse orientation.
> 
> Dan
>