Re:

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Petr Mikulik wrote:
> It could be changed to:
> 	... Please see "help dgrid3d".

That would be exactly contrary to what I was getting at.

dgrid3d is *not* automatically the right thing to use here.  Pointing to 
only it but nothing else would make matters worse than they are now. 
Before, the error message was precisely that: an error message.  It 
pointed out the error.  A message that points out the wrong solution 
would be worse than a message that leaves it for the user to find a 
solution.

The key problem is that a shockingly large number of users encounter 
this error and have no idea whatsoever what "gridded data" means, so 
they don't understand why contouring of non-gridded data might fail. 
The right thing to do is to point them to an explanation of "gridded data".

That's why I recommend strongly to point to "help grid_data" instead of 
dgrid3d.