--- Ernest Berkhout <[email protected]> wrote:
> The idea seems simple, the implementation that I tried so far
> appears to be not... Therefor I firstly report what I have managed
> to tackle myself, but finally there are still some open ends.
> Anyone out there who might have a clue?
>
> The problem is as follows: imagine a twoway (overlayed) graph with
> two lines that cross eachother (maybe more than once), for example
> Y = sin(x) and Y2 = 0.2.
> Now I want to shade the closed areas between these two lines. If
> possible, I also want different colors for the two possibilities
> Y>Y2 and Y<Y2. My first approach was to copy minimum and maximum
> values and then use the -twoway rarea- approach.
>
> ***********************************
> The example can be reproduced with:
>
> set obs 100
> gen x = _n/10
> gen y = sin(x)
> gen y2=.2
>
> *** create low/high-series for the areas where Y>Y2:
> gen low1 = y2 if y > y2
> gen high1 = y if y > y2
>
> *** create low/high-series for the areas where Y<Y2:
> gen low2 = y if y < y2
> gen high2 = y2 if y < y2
>
> twoway (line y x) (line y2 x ) (rarea low1 high1 x ) (rarea low2
> high2 x )
> ********************************
>
> This yields two problems:
> 1. The shaded area of this graph does not extend until the
> exact points of intersection, especially when there are relatively
> few datapoints. The 'edges' of the area are left blank.
> 2. Follows from problem 1: although the variables low1 and high1
> are set to missing when the values of Y are below Y2 (low1>high1),
> there is still an area to plot for Stata, namely from the last
> non-missing observations to the first non-missing observations from
> the next area where Y>Y2 again.
>
> I managed to work around most of the second problem, by setting
> low1 & high1 equal to either Y or Y2 instead of missing. Then the
> code reads:
>
> gen low1 = y2 if y > y2
> gen high1 = y if y > y2
> replace low1 = y2 if low1==.
> replace high1= y2 if high1==.
>
> But when datapoints aren't many (or when functions aren't that
> smooth as in my example), problem 1 still leaves behind its marks.
> What is really needed is a way to adress the graph to fill the area
> up untill the intersection points (and not any further).
> Does anyone have a clue? Is it possible to adress the intersection
> in a continuous way, or does one manually has to compute these
> points and add them (temporarily) to the data? Any opinions are
> welcome!
Ernest,
As far as I know, you have to compute the missing values yourself.
The option -plotregion(margin(zero))- is also useful in this context.
The syntax below seems to do what you want. I changed the first two
commands and the last command.
set obs 101
gen x = _n/10 - 0.1
gen y = sin(x)
gen y2=.2
*** create low/high-series for the areas where Y>Y2:
gen low1 = y2 if y > y2
gen high1 = y if y > y2
replace low1 = y2 if low1==.
replace high1= y2 if high1==.
*** create low/high-series for the areas where Y<Y2:
gen low2 = y if y < y2
gen high2 = y2 if y < y2
twoway (line y x) (line y2 x ) (rarea low1 high1 x ) (rarea low2
high2 x ), plotregion(margin(zero))
Friedrich Huebler
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