Hi – Thanks for trying DataGraph.
By default, the tick mark locations will change depending on the sliding bars and the graph sizing. To specify an increment, try using the Axis settings.
In particular, the tick mark settings sound like what you are looking for.
Note that you may also just want to set the exact size you want for your graph. Then the scaling will not change as you change move the sliding bars.
How to specify the size of a graph
Does this help?