How to create/update an MVAR table using FontTools

[djs-code]

Based on a provided variable font, how would one go about creating and/or updating an MVAR table that updates sTypoAscender and sTypoDescender depending on the value of FVAR axis BLAH? The BLAH axis ranges from 0 to 1000, and the following variations would apply to sTypoAscender and sTypoDescender:

# <axis value>: (<sTypoAscender>, <sTypoDescender>)
variation_data = {
    0: (1424, -426),
    500: (1565, -468),
    1000: (1708, -511)
}

[behdad]

So you have an existing MVAR table that you want to modify? That's a bit tricky.

[djs-code]

Either-or. I can start from not having an MVAR in the first place.

[behdad]

Here's the code in fontTools.varLib that builds the MVAR. Hopefully you can decipher it and achieve what you want.