Используйте plotRangeWithLocation: length:
методы.
-(void)initXYAxesRanges{
//Set graph ranges for x and y planes
CPXYPlotSpace *plotSpace = (CPXYPlotSpace *)graph.defaultPlotSpace;
plotSpace.xRange = [CPPlotRange plotRangeWithLocation:CPDecimalFromFloat(0)
length:CPDecimalFromFloat(10];
plotSpace.yRange = [CPPlotRange plotRangeWithLocation:CPDecimalFromFloat(0)
length:CPDecimalFromFloat(10)];
}