I'm generating some odt/docx reports via markdown using knitr and pandoc and am now wondering how you'd go about formating tables. Primarily I'm interested in adding rules (at least top, bottom and one below the header, but being able to add arbitrary ones inside the table would be nice too).

Running the following example from the pandoc documentation through pandoc (without any special parameters) just yields a "plain" table without any kind of rules/colours/guides (in either -t odt or -t docx).

+---------------+---------------+--------------------+
| Fruit         | Price         | Advantages         |
+===============+===============+====================+
| Bananas       | $1.34         | - built-in wrapper |
|               |               | - bright color     |
+---------------+---------------+--------------------+
| Oranges       | $2.10         | - cures scurvy     |
|               |               | - tasty            |
+---------------+---------------+--------------------+

I've looked through the "styles" for the possibility of specifying table formating in a reference .docx/.odt but found nothing obvious beyond "table header" and "table contents" styles, both of which seem to concern only the formatting of text within the table.

Being rather unfamiliar with WYSIWYG-style document processors I'm lost as to how to continue.

Here's how I searched how to do this:

The way to add a table in Docx is to use the <w:tbl> tag. So I searched for this in the github repository, and found it in this file (called Writers/Docx.hs, so it's not a big surprise)

blockToOpenXML opts (Table caption aligns widths headers rows) = do
  let captionStr = stringify caption
  caption' <- if null caption
                 then return []
                 else withParaProp (pStyle "TableCaption")
                      $ blockToOpenXML opts (Para caption)
  let alignmentFor al = mknode "w:jc" [("w:val",alignmentToString al)] ()
  let cellToOpenXML (al, cell) = withParaProp (alignmentFor al)
                                    $ blocksToOpenXML opts cell
  headers' <- mapM cellToOpenXML $ zip aligns headers
  rows' <- mapM (\cells -> mapM cellToOpenXML $ zip aligns cells)
           $ rows
  let borderProps = mknode "w:tcPr" []
                    [ mknode "w:tcBorders" []
                      $ mknode "w:bottom" [("w:val","single")] ()
                    , mknode "w:vAlign" [("w:val","bottom")] () ]
  let mkcell border contents = mknode "w:tc" []
                            $ [ borderProps | border ] ++
                            if null contents
                               then [mknode "w:p" [] ()]
                               else contents
  let mkrow border cells = mknode "w:tr" [] $ map (mkcell border) cells
  let textwidth = 7920  -- 5.5 in in twips, 1/20 pt
  let mkgridcol w = mknode "w:gridCol"
                       [("w:w", show $ (floor (textwidth * w) :: Integer))] ()
  return $
    [ mknode "w:tbl" []
      ( mknode "w:tblPr" []
        ( [ mknode "w:tblStyle" [("w:val","TableNormal")] () ] ++
          [ mknode "w:tblCaption" [("w:val", captionStr)] ()
          | not (null caption) ] )
      : mknode "w:tblGrid" []
        (if all (==0) widths
            then []
            else map mkgridcol widths)
      : [ mkrow True headers' | not (all null headers) ] ++
      map (mkrow False) rows'
      )
    ] ++ caption'

I'm not familiar at all with Haskell, but I can see that the border-style is hardcoded, since there is no variable in it:

let borderProps = mknode "w:tcPr" []
                    [ mknode "w:tcBorders" []
                      $ mknode "w:bottom" [("w:val","single")] ()
                    , mknode "w:vAlign" [("w:val","bottom")] () ]

What does that mean ?

That means that you can't change the style of the docx tables with the current version of PanDoc. Howewer, there's a way to get your own style.

How to get your own style ?

1. Create a Docx Document with the style you want on your table (by creating that table)
2. Change the extension of that file and unzip it
3. Open word/document.xml and search for the <w:tbl>
4. Try to find out how your style translates in XML and change the borderProps according to what you see.

Here's a test with a border-style I created:

And here is the corresponding XML:

<w:tblBorders>
  <w:top w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
  <w:left w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
  <w:bottom w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
  <w:right w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
  <w:insideH w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
  <w:insideV w:val="dotted" w:sz="18" w:space="0" w:color="C0504D" w:themeColor="accent2"/>
</w:tblBorders>

What about odt ?

I didn't have a look at it yet, ask if you don't find by yourself using a similar method.

Hope this helps and don't hesitate to ask something more