It may not necessarily be true that all numbers need to be less than 7, but I could reproduce the bug only that way.
Steps to reproduce one such graph:
- Go here https://www.mediawiki.org/wiki/Special:GraphSandbox
- Copy and paste the following template in the sandbox {{GraphChart|width=400|height=100|xAxisTitle=X|yAxisTitle=Y|type=rect|x=1,2,3,4,5,6,7,8,9,10,11|y=6,5,2,4,3,1,6,1,2,1,1}} and notice the double values on the Y-axis.