Plot to same graph

[OwenArnold]

Request from user regarding the Reflectometry GUI:

The 'Plot' button generates a new plot each time, which is a bit annoying. Could we have a check if the plot already exists, in which case the plot should just update. Once can always invoke a new plot from the list of workspaces.

There is an option to do this now in Mantid. I believe the window option in plotSpectrum will do what we want.

Further to the above, the users also want to ensure that if they hit process again, and leave the exiting plots up, that they are automatically updated.

[OwenArnold]

This issue was originally trac ticket 10099