引用：http://zjkilly.iteye.com/blog/920956
一句话，设置type属性
"clustered": Values from different series are grouped by category. This is the default type.
"overlaid": Multiple values are rendered on top of one another by category, with the last series on top.
"stacked": Columns are stacked on top of each other and grouped by category. Each column represents the cumulative value of the columns beneath it.
"100%": Columns are stacked on top of each other, adding up to 100%. Each column represents the percent that it contributes to the sum of the values for that category