I have re-classified this as an idea...one that probably also applies to the Pie View, which will display a similar warning if mixing positive and negative values within the same pie.

Meanwhile, the Bar/Line View Cascade visualisation option mixes positive and negative very well....why can't you use Cascade? If Cascade is not appropriate, there should be many ways to selectively transform the data so as not to mix positive and negative values in the same visualisation...