How can I add legend to a multicolored histogram using display?

a:=[1,2,3,4,5]:

if I want b to have all elements less than 4 why can't I just write

b:=a<4;

?

1.I want to plot (x-x0)^2 from x=0..10 with a value for x0 in the plot function not in the procedure. The procedure should 

test x against x0 and if x<x0 then y=0 else y=(x-x0)^2

2.Is it possible to use explore and use x0 as a slider?

Regards

Staffan

defining f:=(t,a,b,c)->a*b/c*t^2 how can I give values for a,b and c inside the plot command?

plot(f(t,a,b,c),t=0..10 ????)

Staffan

Tried different ways to apply unapply but failed:

a := .1994;

modfit3 := a*x^1.5;
 
f := unapply(rhs(modfit3), x);
%;
Error, invalid input: rhs received .1994*x^1.5, which is not valid for its 1st argument, expr

What's wrong here?

S