Incorrect error bar length in bar traces with gaps

[etpinard]

See: http://codepen.io/etpinard/pen/LNoZjJ

More importantly, the benchmark.json mock is currently wrong (and has been wrong for ages) where its error bar are suppressed.

Luckily, @seibs's upcoming PR (currently in seibs#1) fixes this issue.

[etpinard]

fixed in #542