If you need to match the visualization exactly then you may want to consider an extension object. Extension objects are custom object that you can code yourself (or have some one code for you). The world is your oyster with extensions.
I'm sorry if I'm too rough, but I cannot understand why the rounded borders is important in this chart. Even in the last one bar, because of the rounding, you cannot appreciate very well the proportion.
You can represent the same information in QlikView, and also have all the associative experience, search and filtering with an standard combo chart:
You could also use the Stock and Blox Plot representations with this kind of chart.
I know I am a bit stubborn in this requests, but I think the real situation usually is he likes rounded edges, instead of he needs, because I cannot think about any reason apart from the aesthetics where rounded corners has any utility. And I am not sure this reason is worth enough to develop an extension,
I suspect you will spend less time convincing your client than developing a new chart extension.