Android：条形图用achartengine改变y轴刻度

I'm new to achartengine ,and i would like to know how to change the y axis scale. I'm trying to get a bar chart , to display the wifi signal strength and channels .I need for X axis (1,2,3..14) and Y axis (-90,-80,-70...-30) scales. But i don't know where to do that in the code. I'm new to achartengine ,and i would like to kn