Using automatic range will prevent you from changing your range. You can only do this when using a RangeType = Custom and setting RangeMin and RangeMax.
By DataValue+50 I assume that you mean the max value that's coming from the dataset, which you then intend to round to the nearest hundred. If that's the case, you have to go through your dataset and calculate min and max yourself.
chart.Axis.Y.RangeMin = yourMinValue;chart.Axis.Y.RangeMax = yourMaxValue;chart.Axis.Y.RangeType = RangeType.Custom;