fix bug of visualMap that renders linear gradient by mistake
fix bug of visualMap that renders linear gradient by mistake

starlkj/echarts,starlkj/echarts,apache/incubator-echarts,starlkj/echarts,chenfwind/echarts,chenfwind/echarts,ecomfe/echarts,apache/incubator-echarts,100star/echarts,100star/echarts,ecomfe/echarts