Appears to be the same issue expressed as http://www.sencha.com/forum/showthread.php?211228 - As of 3.0.0, the BarSeries cannot handle both positive and negative values. From that assertion, either value is negative, or minY is, which isn't permitted (since both being negative indicates that all bars are negative, and both positive means all are positive). Set a breakpoint on all AssertionErrors and check which value it is that is wrong - there may be a bug in how this assertion is checked, but it seems more likely that some value is negative, or that the minY value has been set too low through some other configuration.
Further debugging shows that the from and to fields for the numeric axis are calculated before there is anything in the data store, which is causing them to be set to NaN. Then went the chart is rendered the minY and maxY are NaN causing this assertion to fail.