Re: multiple plot lines with plot '-'

[<pl...@pi...>]

On 05/16/12 06:56, sfeam (Ethan Merritt) wrote:
> On Monday, 14 May 2012, su...@pi... wrote:
>> On 05/14/12 18:27, Ethan A Merritt wrote:
>>>>>   So could another flag like plot '--' be used to indicate rereading of
>>>>>   the inline data rather than continuing at the next block of inline data ?
>>> This wouldn't work in the general case - you can't reread from a pipe.
>>
>> hmm. I hadn't thought of "inline" data actually being a pipe. Good point.
>>
>> perhaps that could be accepted as a limitation on reading from pipes.
>> It would not reduce the functionality that is already available to pipes.
>
> Up till now, the following have all been equivalent:
>      gnuplot file.in
>      cat file.in | gnuplot
>      cat file.in | gnuplot -
>      gnuplot -e 'load file.in'
>
> I think it would be a bad idea to introduce new syntax that breaks
> this equivalence.

I agree, I like that sort of symmetry.

Part of the problem here is that plot '-' does not work in quite the 
same way as plot datafile

plot '-','-'   plots two consecutive segments from stdin  whereas plot 
datafile , datafile  rereads the file and plots the same thing twice.

This is presumably because you can't reread a pipe but it means plot 
command is acting differently depending on the data source specified.


 >>Consider:
 >>   plot '-' using 1:2, '' using 3:4, '' using 99:100
 >> The first plot uses only columns 1 and 2; these values are stored
 >> internally and could be used again without rereading.  That's what the
  >> "refresh"  command does.  But the 2nd and 3rd plots use data values 
from
 >> columns we didn't use the first time and hence didn't keep.
 >> So those values can only be recovered by rereading.

 > Since this is all in one plot command , couldn't the whole line be
 > parsed to find all the columns to be required by all the using clauses?
 >
 > In fact isn't this already done? Are you implying that
 >
 >   plot datafile using 1:2, '' using 3:4, '' using 99:100
 >
 > actually reads the data file three times looking at different columns :?

 >> Yes.

So if I get this,

gnuplot>  plot datafile using 1:2, '' using 3:4

opens and reads same lines of the file twice. Filling two sets of slots


  gnuplot>  plot '-' using 1:2, '' using 3:4

reads consecutively from stdin , twice. Filling two sets of slots


It would be more efficient if the first version parsed the whole line to 
find all required columns and did one read. I do plots with 5 or 6 
lines, this could be a useful improvement.

Then the suggested '--' could work with no extra effort.
gnuplot>  plot '--' using 1:2, '' using 3:4


The asymmetrical behaviour of the existing plot '-'  would still need a 
different code path.




> If the previous work-around I suggested is too
> long-winded, requiring a "print" and matching quotes on each line,
> how about introducing an idea from perl:
>
> set print 'tempfile.dat'
> print<<  EOF
> 1st line of data
> 2nd line of data
> 3rd line of data
> EOF
>

Yes , I was looking for something like that yesterday. I think that 
would be a useful addition in a number of situations.

However, rewriting all the data in the current file to another 
superfluous copy just to get around the syntax seems a bit clunky.

regards, Peter.

> 	Ethan
>