How do I set a background colour of a chart in the Haskell Chart package?

I've got a complete example here: https://github.com/chrissound/HaskellChartBarGraphExample/tree/backgroundColour

How can I set a background colour of a chart? I've tried fillBackground.

chart :: Bool -> Renderable ()
chart borders = fillBackground (FillStyleSolid $ opaque green)  $ toRenderable layout
...

However it does not seem to have any effect.

Complete source code:

module Main where

import Graphics.Rendering.Chart
import Graphics.Rendering.Chart.Backend.Cairo
import Data.Colour
import Data.Colour.Names
import Control.Lens
import Data.Default.Class

chart :: Bool -> Renderable ()
chart borders = fillBackground (FillStyleSolid $ opaque green)  $ toRenderable layout
 where
  layout =
        layout_title .~ "Sample Bars" ++ btitle
      $ layout_title_style . font_size .~ 10
      $ layout_x_axis . laxis_generate .~ autoIndexAxis alabels
      $ layout_y_axis . laxis_override .~ axisGridHide
      $ layout_left_axis_visibility . axis_show_ticks .~ False
      $ layout_plots .~ [ plotBars bars2 ]
      $ def :: Layout PlotIndex Double

  bars2 = plot_bars_titles .~ ["Cash","Equity"]
      $ plot_bars_values .~ addIndexes [[20,45],[45,30],[30,20],[70,25]]
      $ plot_bars_style .~ BarsClustered
      $ plot_bars_spacing .~ BarsFixGap 30 5
      $ plot_bars_item_styles .~ map mkstyle (cycle defaultColorSeq)
      $ def

  alabels = [ "Jun", "Jul", "Aug", "Sep", "Oct" ]

  btitle = if borders then "" else " (no borders)"
  bstyle = if borders then Just (solidLine 1.0 $ opaque black) else Nothing
  mkstyle c = (solidFillStyle c, bstyle)

main :: IO ()
main = do
  _ <- renderableToFile def "example11_big.png" (chart True)
  return ()

Solution

The default background color of a layout is a solid white. This is somewhat hidden in the documentation of the Default instance of layout: http://hackage.haskell.org/package/Chart-1.9/docs/Graphics-Rendering-Chart-Layout.html

In your example the green background is overlapped entirely by the white background of the layout. The background color of a layout can be modified using the layout_background lens.

Therefore to achieve a green you can either set the backgorund color of the layout to green directly:

layout =
      layout_title .~ "Sample Bars" ++ btitle
    $ layout_title_style . font_size .~ 10
    $ layout_x_axis . laxis_generate .~ autoIndexAxis alabels
    $ layout_y_axis . laxis_override .~ axisGridHide
    $ layout_left_axis_visibility . axis_show_ticks .~ False
    $ layout_plots .~ [ plotBars bars2 ]
    $ layout_background .~ (FillStyleSolid $ opaque green)
    $ def :: Layout PlotIndex Double

or set the backgorund color of the layout to transparent and use fillBackground like you already do:

...    
$ layout_background .~ (FillStyleSolid transparent)
...