I have 2 frames, 1 has continent data, the other metals-group data.
    continents
year  africa  asia
1996    2       3
1997    5       6
    metal-groups
year  ferrous  non-ferrous
1996    2       3
1997    5       6
I need to merge them on 'year', and try keep the categories associated with the columns of the final frame (i.e. continents and metal-groups).
What's the best way to do this?
