Is this included as an email, like how you've included here? Something like this is usually done via javascript. The problem you have is that email clients will rarely, if ever, execute Javascript. The only work around that I can think of is an image that is included on a server (not attached within the email). Then you can use server-side scripts to "talk" with a javascript that will recreate the image.
Really though, this is a lot of upfront work for an email element that would be best if it were just manually adjusted once a month.