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When creating a new chart, or reading a chart in from a file where the minor increment value is not specified, the minor increment value is automatic, which is represented with -1.
This property can be set to any value greater than or equal to zero. If you set this to a negative value, it will set the minor increment of the axis to auto.

If ValueAxis.UseLogarithmicScale is turned on, then this property represent the power of the minor unit in base 10 (or whatever base you've manually set). For instance, if this value is set to 2. Then with UseLogarithmicScale turned off, the resulting file will have a minor unit of 2. If UseLogarithmicScale is turned on, then the resulting file will have a minor unit of 100 (10 ²).