Here is what I would do:
Using Firebug with Firefox, select a chart and look at CSS. Find the name of the border by selecting it with Firebug.
Further:
1.) Create CSS to remove the border and paste the diagram into the DIV with the desired border. Depending on your browser compatibility, you may use images instead of CSS for a rounded border.
OR
2.) Use CSS to change the border to what you want.
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