The circles style plots a circle with an explicit radius at each data point. Three columns of data are required: x, y, radius. An optional 4th column may be used to specify color information. The radius is always interpreted in the units of the plot's horizontal axis (x or x2). The scale on y and the aspect ratio of the plot are both ignored.
Example (draws circles whose area is proportional to the value in column 3):
set style fill transparent solid 0.2 noborder
plot 'data' using 1:2:(sqrt($3)) with circles, \
'data' using 1:2 with linespoints
The result is similar to using a `points` plot with variable size points and pointstyle 6, except that the circles will scale with the x axis range. See also `set object circle` and `fillstyle`.