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You will have to update to 2.8+ version of the library.
There were multiple instances of issues around this on their GitHub (i.e. [BUG] Spacing between vertical bars with percentages set to 1 and [BUG] Small gap between bars even with barPercentage and categoryPercentage set to 1).
There was a PR that made it into their 2.8.0 version that is supposed to fix the issues around this (as noted by @benmccann at the bottom of the PR page).
Source:stackexchange.com