If prior to 8.0, then open Tools->Options dialog and then on Page tab you can change the Margin Control drop-down to Border or Tight and this should remove extra white space. Note that if there is some small object far off the graph layer, Border or Tight would include that object thus including a lot of white space, so first one may need to delete any such object.
If version 8.0, the Export Graph dialog has options right in the dialog to set the "Export Settings" branch and then one could save the settings as a theme for future reuse.
Then you could try resizing/repositioning the layer within the page and export the entire page? Maybe you can send your graph to tech support and we could take a look?