In order to make the Floating Column chart scenario you want, you should use a transparent color for the ColumnSeries that is below the green one from the screenshot , but we are aware of a problem with setting a transparent color in the RadHtmlChart
However, setting the color property won't affect the border of the series items, so a better solution would be to add an Opacity property for the series. This property would control the appearance of all the elements for the series item, so that you can set a transparent color for the border's as well. That is why I have logged this visual improvement in our Feedback portal.
As more and more people vote for it, we will increase the priority of this task and provide it to you as soon as possible.
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