Press Enter in LyX Sweave as You Wish
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For a long time I’ve been wondering why we are not able to use Enter in the LyX Scrap environment which was set up by Gregor Gorjanc for Sweave. Two weeks ago, I (finally!) could not help asking Gregor about this issue, as I’m using “LyX + Sweave” more and more in my daily work. He explained it here: LyX-Sweave: mandatory use of control+enter in code chunks
After digging into the LyX customization manual for a while, I found a solution which allows us to press the Enter key just as we normally do when typing in a LyX document. The key is to use Environment instead of paragraph as LatexType for the style definition of Scrap. Besides, I used the LatexName as wrapsweave, as a LatexName is required by LyX. The definition for wrapsweave is simple: just two empty lines by par. (If you define it as newenvironment{wrapsweave}{}{}, you will run into troubles sometimes; especially when you use indent for paragraphs.)
As we know, LaTeX environment cannot be centered in LyX (only paragraphs can), so I defined a special environment ScrapCenter when I want to insert graphics via Sweave and make them center-aligned.
To put all efforts together, this is the new literate-scrap.inc (compare it with http://cran.r-project.org/contrib/extra/lyx/literate-scrap.inc):
Format 2
Preamble
usepackage{Sweave}
newenvironment{wrapsweave}{par}{par}
EndPreamble
OutputType literate
Style Scrap
LatexType Environment
LatexName wrapsweave
Margin static
TopSep 0.4
BottomSep 0.4
ParSep 0.0
LabelType static
Align left
AlignPossible block, left, right, center
NewLine 0
PassThru 1
FreeSpacing 1
TextFont
Color latex
Family typewriter
EndFont
End
Style ScrapCenter
LatexType Environment
LatexName center
Margin static
TopSep 0.4
BottomSep 0.4
ParSep 0.0
LabelType static
Align center
NewLine 0
PassThru 1
FreeSpacing 1
TextFont
Color latex
Family typewriter
EndFont
End
Enjoy pressing a single Enter key in LyX now! 
P.S. the side effect of this modification is, your R code will be separated by empty lines. But Sweave will remove blank lines by default, so that is not really a big trouble.
P.P.S. Next time I will write my solution of using pgfSweave in LyX — if you have never tried the pgfSweave package in R, I’d strongly recommend you do it right now!! My comment on this package is: amazingly beautiful! See a preview lyx-pgfsweave-demo.
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