diff --git a/R/format.R b/R/format.R
index ec10eb9..8093479 100644
--- a/R/format.R
+++ b/R/format.R
@@ -3,6 +3,8 @@
 #'
 #' @param format Format for numbers, currency, percentage, e.g. \code{".0\%"} for rounded percentage.
 #'  See online documentation : \url{https://github.com/d3/d3-format}.
+#' @param prefix Character string to append before formatd value.
+#' @param suffix Character string to append after formatd value.
 #' @param locale Localization to use, for exemple \code{"fr-FR"} for french,
 #'  see possible values here: \url{https://github.com/d3/d3-format/tree/master/locale}.
 #'
@@ -12,15 +14,15 @@
 #' @importFrom htmlwidgets JS
 #'
 #' @example examples/format.R
-format_num <- function(format, locale = "en-US") {
+format_num <- function(format, prefix = "", suffix = "", locale = "en-US") {
   check_locale(locale)
   path <- system.file(file.path("htmlwidgets/lib/d3-format/locale", paste0(locale, ".json")), package = "apexcharter")
   if (path != "") {
     locale <- paste(readLines(con = path, encoding = "UTF-8"), collapse = "")
   }
   JS(sprintf(
-    "function(value) {var locale = d3.formatLocale(JSON.parse('%s')); return locale.format('%s')(value);}", 
-    locale, format
+    "function(value) {var locale = d3.formatLocale(JSON.parse('%s')); return '%s' + locale.format('%s')(value) + '%s';}", 
+    locale, prefix, format, suffix
   ))
 }
 
diff --git a/examples/format.R b/examples/format.R
index ddecf20..70a9d8a 100644
--- a/examples/format.R
+++ b/examples/format.R
@@ -38,15 +38,23 @@ apex(dat, aes(labels, values), "column") %>%
     formatter = format_num("$,.2f")
   ))
 
-
 # Change locale
 apex(dat, aes(labels, values), "column") %>% 
   ax_yaxis(labels = list(
-    formatter = format_num("$,.2f", "fr-FR")
+    formatter = format_num("$,.2f", locale = "fr-FR")
   ))
 
 
 
+# Customize tooltip value
+# Use SI prefix
+dat <- data.frame(
+  labels = c("apex", "charts"),
+  values = c(1e4, 2e4)
+)
 
-
+apex(dat, aes(labels, values), "column") %>% 
+  ax_tooltip(y = list(
+    formatter = format_num(",", suffix = " GW/h")
+  ))
 
diff --git a/man/format_num.Rd b/man/format_num.Rd
index 90ce8b8..0344ed4 100644
--- a/man/format_num.Rd
+++ b/man/format_num.Rd
@@ -4,12 +4,16 @@
 \alias{format_num}
 \title{Format numbers (with D3)}
 \usage{
-format_num(format, locale = "en-US")
+format_num(format, prefix = "", suffix = "", locale = "en-US")
 }
 \arguments{
 \item{format}{Format for numbers, currency, percentage, e.g. \code{".0\%"} for rounded percentage.
 See online documentation : \url{https://github.com/d3/d3-format}.}
 
+\item{prefix}{Character string to append before formatd value.}
+
+\item{suffix}{Character string to append after formatd value.}
+
 \item{locale}{Localization to use, for exemple \code{"fr-FR"} for french,
 see possible values here: \url{https://github.com/d3/d3-format/tree/master/locale}.}
 }
@@ -60,16 +64,24 @@ apex(dat, aes(labels, values), "column") \%>\%
     formatter = format_num("$,.2f")
   ))
 
-
 # Change locale
 apex(dat, aes(labels, values), "column") \%>\% 
   ax_yaxis(labels = list(
-    formatter = format_num("$,.2f", "fr-FR")
+    formatter = format_num("$,.2f", locale = "fr-FR")
   ))
 
 
 
+# Customize tooltip value
+# Use SI prefix
+dat <- data.frame(
+  labels = c("apex", "charts"),
+  values = c(1e4, 2e4)
+)
 
-
+apex(dat, aes(labels, values), "column") \%>\% 
+  ax_tooltip(y = list(
+    formatter = format_num(",", suffix = " GW/h")
+  ))
 
 }