Radar Charts (also known as spider charts, star charts, star plots, web charts, cobweb charts, irregular polygons or kiviat diagram), can be used in different areas, from sports to economics, in cases of multivariate data, i.e. when we need to compare several parameters of a subject (e.g. how many points did a sportsman get from different hits during his fight). These charts are two-dimensional; one of the axes which holds the values is vertical; the second axis is of ordinal type and it looks like a polygon which vertexes are ticks. Each tick represents a parameter, so the more parameters you compare the more angles does this polygon-axis have. This axis is usually made invisible.
This chart shows us the information about average temperature in London during a year at nights and days. There are twelve categories, each means a month. We can see two polygons here on the radar. The lighter one stands for maximum daily temperatures and the darker means minimum temperatures at nights. The y-axis shows degrees in a range [0,25] with an interval of 5. The x-axis (the one which holds month's names - categories) is shown of the same color as ticks, which makes it visible but not catchy.