I think it´s not possible.
But if you could you would be forcing the user to make a visual comparison that doesn´t make sense since the scale would be different from one chart to another. That could lead to a misinterpretation of the data.
If the indicators scales are different, you should use another kind of chart. Or translate them into the same scale, like '%';
I guess it is a limitation of Trellis' then because ideally many business of Executive Levels would like to see a scorecard of Metrics over time. I understand and appreciate the relativity offered in comparisons but I think it should based on the First Dimension's expression permutation. They can be static over the second dimension but the first being flexible would definitely position Trellis in a win-win situation and provide more uses than just one metric per trellis.