[Feature] get bar gap in renderItem,

### What problem does this feature solve?

to implement this style, use renderItem to get points for each stacked series, the point from `api.coord([xValue, yIndex])` is the center of the bar item, to move it to the bottom of the bar item, division the `barLayout[i].bandWidth` by 2 and minus with y point value, but result is not at the bottom of bar item, but the center of the gap, seems that the `bandWidth` contains the gap, how to minus the gap height?


## see example

[example](https://echarts.apache.org/examples/zh/editor.html?c=custom-bar-trend&code=MYewdgzgLgBAngQQB4EsIBECGVMwLwwDaA5AEwAMpAzAsFMQDQxmUAst9TxAyjmACaYATv2IBdAFAgADlBTh8MAN4SYa9WqTI0ALmWqNhqHGkBTPcQBumADYBXU4wOH1EABYgA7noBmtiKYMzi4wmKgQACoowADWeiohhu5evv6BwS4AvkGJamFoADIoYOb6ua4e3jB-NgE5udkZ6o2JiOHxTRrGZhbA2KYA5iBCcE7lgjh6bWhYOPWJ-ZHRcWXlMMlVNXWdzfOGLSEBQiimEHqEO2oJa2CYALalxADCdtAgd2NrE5jnhORMAEYKEwKP8YACAKzkMRiPaJbqPYCvKDvT7lISmASmIQASSgpjuegAFNJhPcIExMNIUABKfAAPlWaxgoEgIBspgAdDYQAMicRCBisbj8XdQtSxIwYKShOTKdT5ShOdZ7KYieQaTSJJdDKzoDAAEbCAqYOAgOywAhUpVGoQms0WonXZlqUB2MBQKbaDDYTDczEDKBuOENLU6jR69lcnl8gVC_jYvEEw3G03mqCSpi2-3psMulngfXSEDFKAQRSEST5nzDGBEjmwFCKcgAbhgTYAPPBvbM_RywIG3G2UABqEd050uyMc7m8_mCzEJkXJo4nCA4rFITPtvP5guQWBIABqtgcimtytPaqbI_BWr3-6LJY95-pnNAw34RMIx6vTBQYj3nuxaloQAJiDAI4ECmdpphahAAZyRoCAA6ig_BBjAAD0MCkOGhggR6ECctIrxuCSz5QEB-bTtGc5xouiaitKlHboRUD_kwv6qtRayZNq-YYlAdhCGATIugiFjFjYcA2MUjghuU7hUqUk75uxZwsaWFL4RoBz5tAsmqbpSRQEIIAxKUF6WGgdi2PyoA8kIxA0opaw-CgNg2BYYDgApJlqPpfH4UFWRuVc-G3A8FgAApPE8aK5N85xAmCoKAlCsL4ZJzC2oliTQJgsQWBw-X7G5WVZC2QA)