how to use ASIFormDataRequest setPostValue to tag Facebook images

[thistle]

I'm sending an image to Facebook from my iPhone app using the Graph API and ASIFormDataRequest. I want to set tags on the image.

Facebook docs say:

tags: An array containing the users and their positions in this photo. The x and y coordinates are percentages from the left and top edges of the photo, respectively.

Photo - Facebook Developers

If I get info from Facebook on a previously tagged image, the tags look like this:

{
    from = "some name";
    height = 12345;
    
    ...
  
    tags =     {
        data =         (
	         		{
					"created_time" = "some time";
					id = 12345;
					name = "some name";
					x = "41.875";
					y = "35.4005";
				}
			);
	};
}

This is the rest of the request set-up code:

NSURL *url = [NSURL URLWithString:@"https://graph.facebook.com/me/photos"];
ASIFormDataRequest *request = [ASIFormDataRequest requestWithURL:url];
[request addData:UIImagePNGRepresentation(image) forKey:@"data"];
[request setPostValue:msg forKey:@"message"];
[request setPostValue:accessToken forKey:@"access_token"];

Where msg and accessToken are strings.

What should I be passing forKey:@"tags"? An array of a dicts? A dict of arrays of dicts? Some kind of comma separated string?

[request setPostValue:tags forKey:@"tags"];
where tags == ???

Thanks!