diff --git a/NEWS.md b/NEWS.md
index 0438d614b2..3abb02c650 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -119,6 +119,8 @@ core developer team.
 * `stat_bin()` now handles data with only one unique value (@yutannihilation 
   #3047).
 
+* `sec_axis()` now accepts functions as well as formulas (@yutannihilation, #3031).
+
 # ggplot2 3.1.0
 
 ## Breaking changes
diff --git a/R/axis-secondary.R b/R/axis-secondary.R
index 21465ce335..5ab8e1c4c2 100644
--- a/R/axis-secondary.R
+++ b/R/axis-secondary.R
@@ -4,7 +4,7 @@
 #' secondary axis, positioned opposite of the primary axis. All secondary
 #' axes must be based on a one-to-one transformation of the primary axes.
 #'
-#' @param trans A transformation formula
+#' @param trans A formula or function of transformation
 #'
 #' @param name The name of the secondary axis
 #'
@@ -33,8 +33,8 @@
 #' Unlike other continuous scales, secondary axis transformations for date and datetime scales
 #' must respect their primary POSIX data structure.
 #' This means they may only be transformed via addition or subtraction, e.g.
-#' `~. + hms::hms(days = 8)`, or
-#' `~.- 8*60*60`. Nonlinear transformations will return an error.
+#' `~ . + hms::hms(days = 8)`, or
+#' `~ . - 8*60*60`. Nonlinear transformations will return an error.
 #' To produce a time-since-event secondary axis in this context, users
 #' may consider adapting secondary axis labels.
 #'
@@ -43,16 +43,16 @@
 #'   geom_point()
 #'
 #' # Create a simple secondary axis
-#' p + scale_y_continuous(sec.axis = sec_axis(~.+10))
+#' p + scale_y_continuous(sec.axis = sec_axis(~ . + 10))
 #'
 #' # Inherit the name from the primary axis
-#' p + scale_y_continuous("Miles/gallon", sec.axis = sec_axis(~.+10, name = derive()))
+#' p + scale_y_continuous("Miles/gallon", sec.axis = sec_axis(~ . + 10, name = derive()))
 #'
 #' # Duplicate the primary axis
 #' p + scale_y_continuous(sec.axis = dup_axis())
 #'
 #' # You can pass in a formula as a shorthand
-#' p + scale_y_continuous(sec.axis = ~.^2)
+#' p + scale_y_continuous(sec.axis = ~ .^2)
 #'
 #' # Secondary axes work for date and datetime scales too:
 #' df <- data.frame(
@@ -75,12 +75,15 @@
 #' # or to transform axes for different timezones
 #' ggplot(df, aes(x = dx, y = price)) + geom_line() +
 #'   scale_x_datetime("GMT", date_labels = "%b %d %I %p",
-#'   sec.axis = sec_axis(~. + 8*3600, name = "GMT+8",
+#'   sec.axis = sec_axis(~ . + 8 * 3600, name = "GMT+8",
 #'   labels = scales::time_format("%b %d %I %p")))
 #'
 #' @export
 sec_axis <- function(trans = NULL, name = waiver(), breaks = waiver(), labels = waiver()) {
-  if (!is.formula(trans)) stop("transformation for secondary axes must be a formula", call. = FALSE)
+  # sec_axis() historically accpeted two-sided formula, so be permissive.
+  if (length(trans) > 2) trans <- trans[c(1,3)]
+
+  trans <- rlang::as_function(trans)
   ggproto(NULL, AxisSecondary,
     trans = trans,
     name = name,
@@ -142,7 +145,7 @@ AxisSecondary <- ggproto("AxisSecondary", NULL,
   # Inherit settings from the primary axis/scale
   init = function(self, scale) {
     if (self$empty()) return()
-    if (!is.formula(self$trans)) stop("transformation for secondary axes must be a formula", call. = FALSE)
+    if (!is.function(self$trans)) stop("transformation for secondary axes must be a function", call. = FALSE)
     if (is.derived(self$name) && !is.waive(scale$name)) self$name <- scale$name
     if (is.derived(self$breaks)) self$breaks <- scale$breaks
     if (is.waive(self$breaks)) self$breaks <- scale$trans$breaks
@@ -150,12 +153,7 @@ AxisSecondary <- ggproto("AxisSecondary", NULL,
   },
 
   transform_range = function(self, range) {
-    range <- new_data_frame(list(. = range))
-    rlang::eval_tidy(
-      rlang::f_rhs(self$trans),
-      data = range,
-      env = rlang::f_env(self$trans)
-    )
+    self$trans(range)
   },
 
   mono_test = function(self, scale){
diff --git a/man/sec_axis.Rd b/man/sec_axis.Rd
index ad7a9fd737..f89a90c216 100644
--- a/man/sec_axis.Rd
+++ b/man/sec_axis.Rd
@@ -15,7 +15,7 @@ dup_axis(trans = ~., name = derive(), breaks = derive(),
 derive()
 }
 \arguments{
-\item{trans}{A transformation formula}
+\item{trans}{A formula or function of transformation}
 
 \item{name}{The name of the secondary axis}
 
@@ -53,8 +53,8 @@ As of v3.1, date and datetime scales have limited secondary axis capabilities.
 Unlike other continuous scales, secondary axis transformations for date and datetime scales
 must respect their primary POSIX data structure.
 This means they may only be transformed via addition or subtraction, e.g.
-\code{~. + hms::hms(days = 8)}, or
-\code{~.- 8*60*60}. Nonlinear transformations will return an error.
+\code{~ . + hms::hms(days = 8)}, or
+\code{~ . - 8*60*60}. Nonlinear transformations will return an error.
 To produce a time-since-event secondary axis in this context, users
 may consider adapting secondary axis labels.
 }
@@ -63,16 +63,16 @@ p <- ggplot(mtcars, aes(cyl, mpg)) +
   geom_point()
 
 # Create a simple secondary axis
-p + scale_y_continuous(sec.axis = sec_axis(~.+10))
+p + scale_y_continuous(sec.axis = sec_axis(~ . + 10))
 
 # Inherit the name from the primary axis
-p + scale_y_continuous("Miles/gallon", sec.axis = sec_axis(~.+10, name = derive()))
+p + scale_y_continuous("Miles/gallon", sec.axis = sec_axis(~ . + 10, name = derive()))
 
 # Duplicate the primary axis
 p + scale_y_continuous(sec.axis = dup_axis())
 
 # You can pass in a formula as a shorthand
-p + scale_y_continuous(sec.axis = ~.^2)
+p + scale_y_continuous(sec.axis = ~ .^2)
 
 # Secondary axes work for date and datetime scales too:
 df <- data.frame(
@@ -95,7 +95,7 @@ ggplot(df, aes(x = dx, y = price)) + geom_line() +
 # or to transform axes for different timezones
 ggplot(df, aes(x = dx, y = price)) + geom_line() +
   scale_x_datetime("GMT", date_labels = "\%b \%d \%I \%p",
-  sec.axis = sec_axis(~. + 8*3600, name = "GMT+8",
+  sec.axis = sec_axis(~ . + 8 * 3600, name = "GMT+8",
   labels = scales::time_format("\%b \%d \%I \%p")))
 
 }