Hi Daniel,
If the Xmin sometimes is out of the range of what you are expecting and do not want the graph to do that, you should turn off the autoscaling function (Right Click->X Scale->Autoscale). You can do the same thing programmatically using XScale.Scale Fit property in property node. Wire a zero constant to the Scale Fit property to turn off autoscaling. You can also set the minimum value of X scale by using the X Scale.Range.Minimum property.
Regards,
Ankita A.