instead of using plot, try drawing a line from one point to the next, this should give a better result.
Const density#=1.0
x_centre=100
y_centre=100
x_diameter=100
y_diameter=100
For a#=0 To 359 Step density#
x1=x_diameter*Sin(a#)+x_centre
y1=y_diameter*Cos(a#)+y_centre
x2=x_diameter*Sin(a#+density)+x_centre
y2=y_diameter*Cos(a#+density)+y_centre
line x1,y1,x2,y2
Next
Const density#=1.0
x_centre=100
y_centre=100
x_diameter=100
y_diameter=100
For a#=0 To 359 Step density#
x1=x_diameter*Sin(a#)+x_centre
y1=y_diameter*Cos(a#)+y_centre
x2=x_diameter*Sin(a#+density)+x_centre
y2=y_diameter*Cos(a#+density)+y_centre
line x1,y1,x2,y2
Next