Formating a Grid with Reals "."-aligned

Question

I have a table of real numbers and want to format a Grid such that the headlines are centered and the contents are aligned at the decimal point. MWE:

data = Table[RandomReal[{1, 100}], {i, 1, 5}, {j, 2}]

headers:

columns = {"one", "two"};

Rows:

rowsLabels = StringJoin["R", ToString[#]] & /@ Range[5]
rows = Flatten /@ ({rowsLabels, data}\[Transpose])

Table:

tab = Join[{Prepend[columns, "Text "]}, rows]

Then

Grid[tab, 
 Dividers -> {{2 -> True}, {2 -> True}},
 Alignment -> {{Left, Center, Center}, {{Left, ".", "."}}, Baseline}]

delivers:

and

Grid[tab, 
 Dividers -> {{2 -> True}, {2 -> True}},
 Alignment -> {{{Left, Center, Center}, {Left, ".", "."}}, Baseline}]

delivers:

So the second columns is fine, but the first not. Can anyone give me a hint on how I can achieve the desired result.

Answer 1

It looks like using a list specification doesn't work for this. Instead, you can use the 3 element list form of Alignment:

Grid[tab, Alignment->{{Left,".","."},Baseline, {{1,1},{1,3}}->Center}]

Alternatively, you could use Item to override the headers:

Grid[
    MapAt[Item[#, Alignment->Center]&, tab, {1, All}],
    Alignment->{{Left, ".", "."}, Baseline}
]