Heatmap 'select' apply to column instead of cell

### Version
4.9.3

### Reproduction link
[https://jsfiddle.net/2j1bgshz/3/](https://jsfiddle.net/2j1bgshz/3/)

### Steps to reproduce
Click any cell with data inside.

### What is expected?
The cell is selected, with a default gray border displayed.

### What is actually happening?
Besides the clicked cell, all of those in the same column look as if they were selected.

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In the console.log output of 'selectchanged' event, there is an array named selected[0][dataIndex]. It changes after every event. It stores the indexes of all selected cells, instead of columns.
I expect the style of selected cells can change accordingly, however, it goes wild to the whole column.
I think it's a bug.
I tried solving it by putting every row's data in a different series, but that seems not really smart.
Looking forward to better solutions :)