Hi Sathyan,
Try to use the below piece of code to your Class declaration in CSS file and let us know.
.v-datapoint.v-morphable-datapoint{
rx: 15px !Important;
ry: 15px !Important;
}
The pixel value definition is based on your requirement means you can specify any number based on the required rounded corners.
Rgds,
Sethu.
Note : I have seen the above piece working in Chrome Browser but not IE, so do check in Firefox too if you have the Browser to Test.