diff --git a/lib/tk/dataframe.ml b/lib/tk/dataframe.ml
index e11d48e0d26f296d4206b2bba36a66d1d7072392..aaba58a4650f0658fe5cda62921aa64e4db4c269 100644
--- a/lib/tk/dataframe.ml
+++ b/lib/tk/dataframe.ml
@@ -140,12 +140,15 @@ let from_file ?(header = `Read_in_file) path =
   let cols = List.zip_exn labels cols in
   Ok { nrows ; ncols ; cols }
 
-let to_html ?(cell_renderers = []) d =
+type html_formatter =
+  int -> string -> Html_types.td_content Tyxml.Html.elt
+
+let to_html ?(formatters = []) d =
   let open Tyxml.Html in
-  let default_cell s = txt s in
+  let default_cell _ s = txt s in
   let cols = List.map d.cols ~f:(fun (label, col) ->
       let cell_renderer =
-        List.Assoc.find cell_renderers ~equal:String.equal label
+        List.Assoc.find formatters ~equal:String.equal label
         |> Option.value ~default:default_cell
       in
       label, cell_renderer, col
@@ -167,7 +170,7 @@ let to_html ?(cell_renderers = []) d =
     | String_opts t -> Option.value ~default t.(i)
   in
   let row i =
-    List.map cols ~f:(fun (_, renderer, col) -> td [renderer (elem col i)])
+    List.map cols ~f:(fun (_, renderer, col) -> td [renderer i (elem col i)])
     |> tr
   in
   table ~thead (List.init (nrows d) ~f:row)
diff --git a/lib/tk/dataframe.mli b/lib/tk/dataframe.mli
index 826172baa3f4b9f4a5dc612c13553c660760b6d7..c8a8c06d75a79a6fb992f40e900ff382a74c9433 100644
--- a/lib/tk/dataframe.mli
+++ b/lib/tk/dataframe.mli
@@ -24,10 +24,10 @@ val from_file :
   string ->
   (t, [> `Msg of string]) result
 
+type html_formatter =
+  int -> string -> Html_types.td_content Tyxml.Html.elt
+
 val to_html :
-  ?cell_renderers:
-    (string *
-     (string -> [< Html_types.td_content > `PCDATA] Tyxml.Html.elt))
-    list ->
+  ?formatters:(string * html_formatter) list ->
   t ->
   [> Html_types.table ] Tyxml.Html.elt