Help needed: vowels as diacritics

Hi there! Please forgive me for asking such a noob question, but I’m not a typographer, I design conscripts.
I’m making a writing system where lowercase vowels (also some consonants and ligatures) are presented as diacritics above the preceding letter. I’m having trouble with Glyphs, as I cannot seem to make my “a” appear as a dot above the previous letter. I’ve changed the category of “a” to be a mark, I’ve added the _top anchor to the glyph and it doesn’t show up.

You need to write the according feature code into mark feature.

Well, I solved it. I didn’t realize that Glyphs does not show the preview for marks. So, even if you do everything correctly, the mark will be shown in exported font only.

You can get a preview with a custom GlyphData.xml.

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Or use the “Text Preview” window.