set object <index> ellipse {at|center} <position> size <w>,<h>
{angle <orientation>}
{<other-object-properties>}
The position of the ellipse is specified by giving the center followed by the width and the height (actually the major and minor axes). The keywords `at` and `center` are synonyms. The center position may be given in axis, graph, or screen coordinates. See `coordinates`. The major and minor axis lengths must be given in axis coordinates. The orientation of the ellipse is specified by the angle between the horizontal axis and the first axis of the ellipse. NB: If the x and y axis scales are not equal, then the major/minor axis ratio will no longer be correct after rotation. This may be fixed in a later gnuplot version.
Note that `set object ellipse size <2r>,<2r>` does not in general produce the same result as `set object circle <r>`. The circle radius is always interpreted in terms of units along the x axis, and will always produce a circle even if the x and y axis scales are different and even if the aspect ratio of your plot is not 1. 'Set object ellipse' interprets the first '2r' in terms of x axis units and the second '2r' in terms of y axis units. This will only produce a circle if the x and y axis scales are identical and the plot aspect ratio is 1.