Adding a fix for the inability to set the point lineWidth to 0

This is a bit of a hack taken from issue https://github.com/flot/flot/issues/842

One option is to set the point lineWidth very low as this patch does, and the other is to set the color to rgba(0,0,0,0). If the alpha option is used, then the point width is still technically set to 0, and the end result is that the shadows are lost. With the way this fix is implemented, the shadows can still be set to either 0 or a higher value.

Hope this is useful!