Bar chart overcounts?

[benknoble]

https://github.com/ralsei/graphite/blob/6155173645965ad5a102b2d96f1bef79f82c6b6f/graphite-lib/bar.rkt#L68

I think this line of code makes the hash-table overcount, reporting 10 observations if there were only 9, for example, since add1 is applied to the default 1 when inserting the first value. Changing to 0 should be correct.