Re: Feature request - assignment in expressions

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

On Monday 07 May 2007 23:56, pl...@pi... wrote:
> 
> thanks very much for picking this up. That seems quite close to what I was  
> requesting. I extended your example to pick up the x value of the ymax as  
> well. The plot gives the sample-and-hold plot as expected. Proof of  
> principle. Nice one.
> 
> The nice thing is this can be limitted by setting the range before plot.  
> This is a nice quick solution that avoids quite a bit of effort messing  
> about with external scripts for simple stuff like this. A nice addition.

Please continue to play with it.  We need to discover what limitations and
drawbacks this simple implementation may have before seriously considering
to add some variant to the program.

I hope we can come up with a nicer syntax, as it seems ugly to me to hide
this inside the 'using' clause.  Thinking out loud....  what about a 
parallel clause with a name like 'after'.  The operations in the 'after'
clause would be carried out before passing the data through for actual
plotting:

   plot <foo>  using 1:2 after track_max($2) with lines

perhaps it should allow multiple operations:

   plot <foo> using 1:2 after {track_max($2), track_min($2), sum($2)} with lines


More thoughts:

- (Pro) As compared to filtering through an external script, this mechanism
  has the advantage that it can affect internal gnuplot user variables.

- (Con) This is incompatible with the revised zooming/refresh code
  that I am working on.  There the idea is to avoid re-reading the input
  data if all you want to do is replot it.  But the assign/after mechanism
  is applied during data input, so it would be bypassed by any such shortcut.

- Having to use the name of the variable rather than the variable itself
  may make sense to a low-level programmer, but is likely to be confusing
  to many users.  Suggestions for a syntax that hides this level of 
  complexity would be welcome.  Or perhaps some additional cleverness in
  the parset is possible.  We have one such function already,
  exists("VARNAME"), but that one is also confusing.

> This presents me with some small issues remaining to be solved:
> 
> 1. My xdata is time format , I imagine your patch does not try to fit all  
> cases but in assigning the x value it gets truncated to the nearest hour,  
> 12:45 becomes 12.0

I have never understood the time-handling code.  I suspect that the
whole special case time format mechanism is no longer needed, as the same
goal can be achieved using general string-handling functions.
In your case, you probably need to use some combination of stringcolumn()
strptime() strftime() rather than setting a time format.

> 2. my inexperience with gnuplot, it seems that if I try to set a label  
> after the plot command (which I need to do for xmax,ymax) it does not  
> show. The same command before plot command does work but it's at (0,0).

Of course.  When you draw a new graph by issuing the command "plot" or
"splot", it uses the currently defined labels.
If you define a label afterwards, how could it possibly appear in the
already-drawn plot?  If you don't mind reading the data a second time,
you could "replot" to pick up the new labels.


-- 
Ethan A Merritt