Negative Axis Charts - ExtJs 4

I've a case in which the chart data has negative values to be plotted. ExtJs is plotting the negative values in the chart without giving any config change. But it doesn't look fine { as shown in attachment 1 }.

I've added the config as "minimum : -20" and got the desired chart { as in attachment 2}.

Now the problem is, I've given the minimum config but I dont have any negative values to be plotted then the whole thing gets awkward shifting the axis upwards {as in attachment 3 }

My requirements & queries are :

1. Is there a way that ExtJs would act dynamically and place the negative axis only if data has negative values.

2. How to determine the least minimum of all data points that comes for plotting to give in the "minimum : " configuration.

Please help me out.. Thanks and regards,

Satya Dileep Kumar Thotakura.

Chennai.