Android : achartengine heap chart display bars when series value is equal to zero

I recently started using achartengine 1.2.0 for new type of chart, the HEAP one. It works great in general but I've found a one issue with the way the chart handles zero values in series. I do have a chart with 7 bars (each bar is one day of week) and multiple series. I noticed that values in series which are equall to zero are still rendered on chart as a tiny line. Here's the screenshot:

How I can get rid of those lines?