Query for a member of 'constantCoordinateValue' in something not structural or combined error in the main diagram

I'm trying to draw a histogram for my application using the main plot, as let's get here
 http://code.google.com/p/core-plot/source/browse/examples/CPTestApp-iPhone/Classes/CPTestAppBarChartController.m?r=1dcee044c4981113f38b3445880c88e14bbb

I get an error for

CPXYAxisSet *axisSet = (CPXYAxisSet *)barChart.axisSet;
CPXYAxis *x = axisSet.xAxis;  
x.constantCoordinateValue = CPDecimalFromString(@"0");  

for the above line

error: query for memberCoordinateValue constant for a member that is not a structure or union, and the same error for the following

y.constantCoordinateValue = CPDecimalFromString(@"0");  

I hope some people know how to solve this problem.

early