Chart Macro doesn't display the generated image but a URL instead

Here's how it works:

• The XWiki 2.0 chart macro uses javax.servlet.context.tempdir to generate the chart image
• The ChartingAction used to display the image uses first xwiki.temp.dir and if not defined then javax.servlet.context.tempdir

Thus if xwiki.temp.dir is set the chart macro generates image will never be found unless xwiki.temp.dir points to the same location as javax.servlet.context.tempdir