Improvement/bartooltip item with text span children

[jrc2139]

redo of #544

This is a hold over for #72 in that you are able to better customize the text on the tooltip. I tried to change the code as little as possible by making the List field optional.

[imaNNeo]

Why did you put List around [ ] ?

[jrc2139]

i figured this is the easiest way to do it without breaking any compatibility. could have also done named. i have no preference

[jrc2139]

so you can still call BarTooltipItem('something', TextStyle(fontSize: 12)) with no issue

[imaNNeo]

Okay please change it to named parameter.
Also, it will be great if you do the same in LineTooltipItem, and ScatterTooltipItem.

[imaNNeo]

Also please update one of our samples and use TextSpan (without adding any content like your gif).
Just add a new text using span instead of our previous method of adding texts.
For example, You can leave Saturday same as previous. and instead of the number below it. Add a customized TextSpan.
(I mean don't change any content)

[imaNNeo]

And the last step is updating the documents.
Update the bar_chart.md, scatter_chart.md, and the line_chart.md.

Thanks!

[imaNNeo]

And the last step is updating the documents.
Update the bar_chart.md, scatter_chart.md, and the line_chart.md.

Thanks!

Hi. I think you forgot to update the docs.
Also, you have a conflict.

[jrc2139]

opening again for null-safety merge