If different variables on a single graph require different scales, they should be scaled against a common baseline index, rather than showing multiple scales.
Additional Information:
Rather than showing power in megawatts and profits in dollars, both might be graphed in terms of percent change from a baseline. An indexed chart can permit comparisons among different variables when multiple scales would otherwise be needed. However, care should be taken in selecting an appropriate baseline against which to index, in order to ensure that comparisons will not be biased. Index scaling may also be appropriate for showing the effect of a single variable whose units of measurement change in real value with time.