fix issue with bar relative mode when first bucket is 0 or string

[keeganmccallum]

No description provided.

[mdtusz]

Instead of coercing to a string, could we instead just ensure a more robust check on the case of -0?

function isPositive(x){
  return 1 / x === 1 / Math.abs(x);
}

isPositive(0) // true
isPositive(-0) // false

If you could also add a test or two for this, that would be great! Thanks for the contribution!

[etpinard]

@keeganmccallum I cherry-picked your commits onto the fix-bar-relative-first-bucket to add a few tests using a testing routine introduced in #716 .

I'll close this PR and make a new one (that includes your commits) as soon as #716 is merged.

Thanks again for your efforts.