They arn't. Stephen Few wrote a really good book a few (ha!) years back called 'Information Dashboard Design' (0596100167) which addressed the issue of circular

They arn't. Stephen Few wrote a really good book a few (ha!) years back called 'Information Dashboard Design' (0596100167) which addressed the issue of circular dials and gauges. Here is a good essay on the subject by Stephen Few (http://www.perceptualedge.com/articles/08-21-07.pdf http://www.perceptualedge.com/articles/08-21-07.pdf). Basically, it boils down to the same arguments put forward by Tufte et al. It is far easier to compare horizontal or vertical positions than radial values.