ihaskell-diagrams: make imgHeight configurable

ihaskell-display/ihaskell-diagrams/IHaskell/Display/Diagrams.hs

 {-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-}
 
-module IHaskell.Display.Diagrams (diagram, animation) where
+module IHaskell.Display.Diagrams (diagram, animation, imgHeight) where
 
 import qualified Data.ByteString.Char8 as Char
 import           System.Directory
 import           System.IO.Unsafe
+import           Data.IORef
 import           Diagrams.Backend.Cairo
 import           Diagrams.Prelude
 import           IHaskell.Display
@@ -16,26 +17,31 @@ instance IHaskellDisplay (QDiagram Cairo V2 Double Any) where
     svg <- diagramData renderable SVG
     return $ Display [png, svg]
 
+{-# NOINLINE imgHeight #-}
+imgHeight :: IORef Double
+imgHeight = unsafePerformIO (newIORef 300)
+
 diagramData :: Diagram Cairo -> OutputType -> IO DisplayData
 diagramData renderable format = do
   switchToTmpDir
 
+  imgHeight' <- readIORef imgHeight
+
   -- Compute width and height.
   let w = width renderable
       h = height renderable
       aspect = w / h
-      imgHeight = 300
-      imgWidth = aspect * imgHeight
+      imgWidth = aspect * imgHeight'
 
   -- Write the image.
   let filename = ".ihaskell-diagram." ++ extension format
-  renderCairo filename (mkSizeSpec2D (Just imgWidth) (Just imgHeight)) renderable
+  renderCairo filename (mkSizeSpec2D (Just imgWidth) (Just imgHeight')) renderable
 
   -- Convert to base64.
   imgData <- Char.readFile filename
   let value =
         case format of
-          PNG -> png (floor imgWidth) (floor imgHeight) $ base64 imgData
+          PNG -> png (floor imgWidth) (floor imgHeight') $ base64 imgData
           SVG -> svg (Char.unpack imgData)
 
   return value