diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,12 @@
 # Changelog for vega-view
 
+## 0.3.1.0
+
+The default port is 8082 but this can be changed by setting the PORT
+environment variable before running the server. An invalid setting, or
+one already in use, will cause the application to fail with a less-than
+graceful error message.
+
 ## 0.3.0.5
 
 Improved error handling when displaying Vega or Vega-Lite specifications,
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -20,6 +20,8 @@
  - and will list the files in a given directory and, when selected,
    view them inline or on a separate page.
 
+![A Vega-Lite specification being dragged from a file browser and dropped onto the index page of the vega-view web server](https://raw.githubusercontent.com/DougBurke/vega-view/master/images/vega-view-drag-n-drop.gif)
+
 ## License
 
 This is released under a BSD3 license.
@@ -28,10 +30,14 @@
 
 The server - called `vega-view` - should be run from the directory
 containing the specifications to view. It then provides a web server
-on port 8082 that can be used to view them at the URLs
+on port 8082 (unless the `PORT` environment variable is set, in
+which case this value is used instead) that can be used to view them
+at the URLs
 
     http://localhost:8082/
     http://localhost:8082/display/
+
+(where `8082` should be replaced by the value of `$PORT` if set).
 
 The first page lets you drag-and-drop files onto the page to view
 them. Thse second lets you browse the visualizations that are present
diff --git a/app/Main.hs b/app/Main.hs
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -1,14 +1,30 @@
 {-# LANGUAGE OverloadedStrings #-}
 
 {-
-A very-basic viewer for Vega and Vega-Lite specfications. You
-can browse from the current working directory, and view any
-individual specification via Vega Embed (or an error if it isn't
-a JSON file or some other issue).
 
-A drag-and-drop page could be added, as could and endpoint to which
-you post a specification.
+Usage:
 
+   ./vega-view
+
+will create web pages at
+
+  http://localhost:n/
+  http://localhost:n/display/
+
+where n is 8082 unless the PORT environment variable is set to an
+integer, in which case that will be used.
+
+The top-level page can be used to drag-and-drop specifications and
+view them, and supports several modes:
+
+  - add to start
+  - add to end
+  - only show the current visualization
+
+whereas the display/ directory lets you view any Vega and Vega-Lite
+specfications in the working directory (or sub-directories), either
+"in line" (i.e. in the page) or as a separate page.
+
 The code could be refactored to be a SPA, but does it need to be?
 
 -}
@@ -33,6 +49,7 @@
 import Data.Version (showVersion)
 import Network.HTTP.Types (status404)
 import System.Directory (doesDirectoryExist, listDirectory)
+import System.Environment (lookupEnv)
 import System.FilePath ((</>), takeDirectory, takeFileName)
 import Text.Blaze.Html5 ((!))
 import Text.Blaze.Html.Renderer.Text (renderHtml)
@@ -751,4 +768,10 @@
 
 -- for now assume current directory  
 main :: IO ()
-main = scotty 8082 webapp
+main = do
+  mPortStr <- lookupEnv "PORT"
+  let port = case read <$> mPortStr of
+               Just n -> n
+               _ -> 8082
+
+  scotty port webapp
diff --git a/vega-view.cabal b/vega-view.cabal
--- a/vega-view.cabal
+++ b/vega-view.cabal
@@ -1,14 +1,14 @@
 name:           vega-view
-version:        0.3.0.5
+version:        0.3.1.0
 synopsis:       Easily view Vega or Vega-Lite visualizations.
 description:    A web server that is used to view all the Vega and Vega-Lite
                 specifications in a directory, or sub-directory. It is similar
                 in overall scope to the
                 <https://github.com/vega/vega-desktop vega destkop>
                 tool. Please see the README on GitHub at
-                <https://github.com/DougBurke/vega-view#readme>
+                <https://github.com/DougBurke/vega-view#vega-view>
 category:       Visualization
-homepage:       https://github.com/DougBurke/vega-view#readme
+homepage:       https://github.com/DougBurke/vega-view#vega-view
 bug-reports:    https://github.com/DougBurke/vega-view/issues
 author:         Douglas Burke
 maintainer:     dburke.gw@gmail.com