improve legend styling

Top-align the right part (usually both sides are about equal in height,
but it would get ugly in case lines are wrapped).

Also increase legend label margin (also in case of wrapping), because
the actual width is 6.2em + 4px in padding.  A more correct solution
would be CSS3 border-sizing:border-box, but not really worth it in this
case (stupid prefixes, not to mention msie boycotting anything new).