How to partially change color of each bar in bar chart?

[robert90]

I'm using Qt 5.11.3.

I intend to draw a bar chart like this:

The chart will display bars which have the bottom part with another color (e.g. black in this snapshot).
Also, for each bar, I can change the height of the bottom part for any event e.g. every interval....

Can anyone help to give hints or the approach for this. I'm just newbie with Qt.

Thanks

[robert90]

any suggestion?