One of the classic references on good practices in statistical graphics, The Visual Display of Quantitative Information (2nd ed.) by Edward Tufte, recommends: "The number of information-carrying (variable) dimensions depicted should not exceed the number of dimensions in the data." (p. 71).
According to this principle it would be better to represent the weights by the lengths of rectangles (of fixed height), as you've done it already, rather than by the areas of squares. (E. Tufte shows a graph with circles whose sizes represent city populations as a bad example.) Indeed, it's easier for the human eye to recognize that one rectangle is, e.g., twice as long as another rectangle than to see that one square has twice the area of another square (i.e. a side length that is 1.4142... times as large).
As to the confidence interval of the overall population, I would depict it in a similar way as the others, maybe with thicker lines or in a different color. With a (slightly) stretched diamond it might be less obvious that the diamond width has that particular meaning.
To get rid of the colors you could add style=journal after ods html. If you want to fine-tune the grey scales (cf. http://www.amadeus.co.uk/sas-training/tips/6/1/99/50-shades-of-grey.php), I'm sure we could find a way to do this, too.
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