By default, your function is drawn over the x range
0 to 1. This is explicit in the help.
So for example
. twoway function y = x^2, ra(-3 3)
will show the symmetry of the parabola around
the y axis.
Some further hints are contained in
Stata tip 15: Function graphs on the fly.
Stata Journal 4(4): 488--489 (2004)
Nick
n.j.cox@durham.ac.uk
absc11@bangor.ac.uk
> When I try to draw a symemtric function about the y-axis, e.g.
> function y = x^2
> function y = cos(x)
> function y = tan(x)
> .etc
>
> then I get only the half of the plot, the positive (x,y)'s.
> Also, the drawing of the plot stops at some arbritary point,
> which I imagine is
> based on the number of points drawn.
>
> How can I draw a symmetric parabola about the y-axis, and how
> can I impose a
> more extend drawing of the function.
>
> My specific interest has to do with plotting the upper limit
> of distribution
> functions for skewness and kurtosis combinations, i.e. (b2 -
> b1 =1), when
> skewness is expressed in square roots.
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