When adding a circle object to a plot that uses logscale xy or logscale y, size requires two arguments when the reference is 'first', e.g.
set object circle center first 1,1 size first 0.1
will not work ("rect has y coord of 0; must be above 0 for log scale!"), while
set object circle center first 1,1 size first 0.1,0.1
works. The documentation specifies only one argument (the radius) to size.
(Attachement: Test case)
Diff: