Protovis, like Tableau, are based on the grammar of graphics framework. In a nutshell, in both environments, a chart designer can map visual attributes (such as x or y dimension, color, shape, etc.) to dimensions of data.
The flat file which Becker’s Barley is based on can be used in Tableau public nearly as is.
Here’s a size-by-size comparison of the results:
How it’s done in Protovis
In protovis, the flat file is nested several times, so that its various elements can be called in a hierarchy from series of dots per variety, to panel per sites to a grander panel. Legend and ticks are added by hand for a perfect finish. Still, some careful planning is required to prepare the data file and to adjust the various elements (choice of colors, sizes, etc.)
How it’s done in Tableau
The data file, unsurprisingly, has 4 dimensions: site, variety, year and yield. In tableau terms, yield is a measure (a numerical dimension) while the other 3 are categorical dimensions. With a few clicks, it is possible to get a result which is similar to the original vis in Protovis. We assign yield to column (horizontal attribute), site and variety to row in that order. We also untick aggregate measures in Analysis, so we get little circles and not big bars. Here, I’ve manually sorted the sites and the varieties.
It is much, much easier to achieve a similar result with Tableau, however using protovis provides a finer control.