clean up for plot.txt used in texinfo

[John W. Eaton]

On 17-Nov-2007, Joseph P. Skudlarek wrote:

| The info file generation would complain 
| 
| makeinfo -I.. -I. -I./.. ./octave.texi
| .//plot.texi:68: warning: @image file `plot.txt' (for text) unreadable: No such file or directory.
| .//plot.texi:311: warning: @image file `hist.txt' (for text) unreadable: No such file or directory.
| .//plot.texi:587: warning: @image file `errorbar.txt' (for text) unreadable: No such file or directory.
| .//plot.texi:749: warning: @image file `polar.txt' (for text) unreadable: No such file or directory.
| .//plot.texi:984: warning: @image file `mesh.txt' (for text) unreadable: No such file or directory.
| .//plot.texi:1007: warning: @image file `plot3.txt' (for text) unreadable: No such file or directory.
| 
| To fix this, and to have improve the look of the plots in the info file,
| please consider the following patch.
| 
| doc/ChangeLog Entry
| 
| 2007-11-16  Joseph P. Skudlarek  <Jskud at Jskud.com>
| 
| 	* interpreter/Makefile.in: Add <category>IMAGES_TXT macros, use
| 	them uniformly, and populate PLOTIMAGES_TXT.
| 
| 	* interpreter/plotimages.m: Generate _plot_.txt file for the
| 	texinfo @image command to process.
| 
| 	* interpreter/sparseimages.m: Add initial blank line to txt plots
| 	so they reproduce better in info files.

I applied this patch.

Thanks,

jwe