Hello Sreeraj,
it seems that you are using an older version of the Pie chart implementation, which was based on the rotation of an "pie" image. This implementation has some inaccuracies at the borders - as you already noticed.
Therefore we improved the implementation beginning with version 11 and changed to vector graphics.
Let me recommend to use the latest version. alternatively, as some kind of workaround, it might help to put another pie chart (with a full colored circle) in the background so that the black color does not shine through the borders.
I hope that helps.
Best regards,
Manfred.