Proper math minus signs on the y-axis

[dreeves]

Desiderata

• Deploy it to graph.beeminder.com
• Deploy it to Beeminder proper
• Tweet the UVI

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We'll get this for free when we upgrade from D3 version 5 to version 7 but I ended up putting in a patch, mainly so quals wouldn't fail when refactoring.

Before/after:

Cognata

• Upgrade D3 to version 5 #68

Verbata: typography, nitpickery, unicode, mathematical minus sign, hyphen,