Re: irreversibly reversing range

[sfeam (E. Merritt) <eam...@gm...>]

On Sunday, 09 September 2012, Daniel J Sebald wrote:
> This morning I came to wonder, What if one wants one end of the range 
> fixed while the other is autoscaled?  So I tried a few things, and the 
> results certainly aren't intuitive, or are buggy.
> 
> Say, for example, my intention is to make a plot for which the left side 
> starts at x=30 and is autoscaled on the right side.  I try
> 
> gnuplot> plot [30:*] x
> 
> and get something very unexpected, unless I really think about it. 

Autoscaling doesn't have an obvious meaning for function plots.
Normally that command would be followed by 'plot <datafile>', with
the meaning that points with x<30 would not be plotted.

> First, the result is reversed, slightly unexpected.  Then I wonder, 10 
> is the "upper" range?  OK, now I see.  If I were to "plot x", the lower 
> range is 0 and upper range 10 by definition.  

Not quite.  The default range for function plots is [-10:10]

> So that is where the 
> reverse axis is coming from.  That means if one does
> 
> gnuplot> plot [3:*] x
> 
> it comes out non-reversed.  And yes that is the result.  OK, perhaps 
> gnuplot should be changed so that the upper limit is "rangemin + 10", 
> and the symmetric case "rangemax - 10".
> 
> Out of curiosity I try
> 
> gnuplot> set xrange [3:*] reverse
> gnuplot> plot x
> 
> and the graph changes.  However,
> 
> gnuplot> set xrange [30:*] noreverse
> gnuplot> plot x
> gnuplot> set xrange [30:*] reverse
> gnuplot> plot x
> 
> brings no change in the graph.  Perhaps that is just a ramification of 
> the "rangemin + 10" issue.
> 
> So, I wonder how I can get better control of the range settings and 
> achieve the initial goal of left end of plot fixed at thirty and the 
> right end autoadjust.  How about
> 
> gnuplot> plot [30:40<*] x

None of the above test commands make sense, IMHO.
Function plots require a complete range specifier.
I'd be in favor of giving an error message in each of those cases.
That is, disallow '*' in the range specifier for function plots.

<aside>  
As you probably recall, my opinion is that a range specifier in the plot 
command is always a bad idea.  If it were just me, I'd remove that option
altogether.  The only reason I can see to allow it is if it is changed so
that the in-line range applies only to the immediately following plot clause.
That way you could get separate ranges for separate plots:
   plot [10:20] f1(x), [20:30] f2(x), [30:100] f3(x)
I don't really like that either, but at least it would provide a 
capability that isn't addressed by simply doing "set xrange" first.
</aside>
  
> I would think that "reverse" could 
> be viewed as "do all the range computations, then when all done reverse 
> the range".

That is indeed what it now means. At least that's the intent. 
Do you have an example that works otherwise?

> In any case, I have a patch all set for Octave to address the proposed 
> mod in 4.7 beta, but I'm reluctant to apply it until 4.7 becomes an 
> official release.  Perhaps we could address the bugs I described above 
> with the current proposed 4.7 behavior in mind and then reassess how 
> one-side-autoadjust works.  That is, fix things so that:
> 
> [a:*] - The autoadjust value * is greater than a
> [*:b] - The autoadjust value * is less than b

That's what it does now in 4.7, at least for me.
Again, do you have an example of data-driven autoscaling that
gives some other result?