If you construct your matrix using the Matrix() function you can call in variables as arguments directly. For example:

```
x1=-7.28;
x2=13.88;
mat = matrix({x1,-1.99, 3.3, 8.59, x2})`
//notice the need to transpose; the syntax is easier to construct this way
```

Here's a more complete building the entire matrix dynamically:

```
Names Default To Here( 1 );
x1=100000; x2=1000000; x3=10000000;
dt = Open( "$SAMPLE_DATA/PopAgeGroup.jmp" );
obj = dt << Bubble Plot(
X( :"Portion 0-19"n ),
Y( :"Portion60+"n ),
Sizes( :Pop ),
ID( :Country ),
Coloring( :Pop )
);
obj << Color Levels( matrix({x1,x2,x3})` );
```

I hope this helps!

@julian