GtkTextTags and paragraph spacing

I’m formatting the contents of a GtkTextBuffer using GtkTextTags, and in general this works very nicely. But there’s one thing I’d like that I haven’t figured out. Here’s the context:

...end of previous paragraph

Heading

Start of first para under heading...

The vertical white space between the previous para and the heading is fine: I don’t want to increase it. But I’d like to reduce somewhat the space between the heading and its following text. The obvious way of doing that would be to give a negative value for the “pixels-below-lines” property of the tag applied to the heading, but that’s not allowed by the API. Is there an alternative that I’m missing, please?

Oops, sorry for the noise. A negative “pixels-below” value is not needed, since the default is not zero. My bad, but this would be clearer if the default value were stated in the API doc.