Question: How to set plot-range programmatically?

How to turn the "a" into a number before it gets plotted? Here's how not to do it:

 

eval(plot(x, x = 0 .. a), a = 3);
Error, (in plot) expecting a real constant as range endpoint but received a
subs(plot(x, x = 0 .. a), a = 3);
Error, (in plot) expecting a real constant as range endpoint but received a

 

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