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A polar chart is one based on the polar coordinate system (as opposed to, for example, the Cartesian coordinate system). Each data point is defined in terms of a coordinate pair (r, theta); r is the distance from the centre of a circle (usually the origin of the polar graph), and theta is the relative angle from a specified reference vector based at the centre of the same circle and extending to the "3 o'clock position" on the circle.
The term 'cluster' has been coined especially for this document This form of data presentation is extremely common in certain disciplines. It defies many tenets of graph drawing, but it offers a useful visual method of imparting information. The most common application is the situation of low, normal and high values; for example, the levels of TSH in subjects whose levels of thyroxin are too high, too low or normal. This representation is common where the levels of an analyte are too low to be measured accurately, or too high for the exact concentration to be relevant and results are quoted as 'greater than some level'. Results are clustered together with the X-axis being descriptive rather than numeric.
This sort of chart is often seen in medical and biological work, but the facility is not offered by many packages. The requirement for this sort of presentation should always be discussed early in an analysis of requirements: it cuts down the choice of packages dramatically and can save a lot of time!
A vector chart is used to display the location, direction and magnitude of XY data pairs