[Which property sets length of each bar in Bar Chart and how to export bar chart to image file from .NET code behind]

We are exporting bar chart to a pdf file and image file.

chart1.OpenData(COD.Values,1,4); 

Chart1.Value[0,0] = 6.45875;

Chart1.Value[0,1] = 6.02853 ;

Chart1.Value[0,2] = 5.93027;

Chart1.Value[0,3] = 5.90892;

chart1.CloseDate(COD.Values);

Now, Chart1.Export(FileFormat.png,@C:\Project\rep.png);

The bar chart is unpredictable. Length of each bar in bar chart is not coming properly.(Expected results)

What does the following statements do? 

Chart1.AxisY.Step = 2.0;

Chart1.ConstantLine[1].Value = 4.5;

Are these properties responsible for proper display of length of each bar?

Please assist. I would appreciate your help !!