st: yrescale cancels yscale(range()) in graphs by a categorical variable

[Toby Robertson <toby.robertson@hotmail.com>]

I want to create a set of plots of y on x according to values of a categorical variable c. I would like each plot a) to have its own scale for the y-axis, but b) to begin from zero.The basic syntax 
- twoway scatter y x, by(c)
creates separate graphs with a common y-axis scale, not necessarily starting from zero.
To achieve (a) a separate y-axis scale for each graph (but not necessarily starting from zero) I can use:
- twoway scatter y x, by(c, yrescale)
To have (b) each y-axis start from zero (but with a common scale for all of them) I can use:
- twoway scatter y x, by(c) yscale(range(0))
However, the combination of the two does not achieve (a) and (b).  The -yscale(range(0))- option has no effect.
- twoway scatter y x, by(c, yrescale) yscale(range(0))
What to do? This must be a common enough requirement. Without understanding Stata's graphic object model too well, I think there ought to be a parameter in the yrescale option such as yrescale(min(0)) that would let me do this easily, but there is not. A workaround involving combining separate graphs by category seems tedious.
Toby Robertson 
 		 	   		  
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