Tooltip template as ContentChild on BaseChart

I’d like to be able to customize the tooltip for any chart that extends BaseChart.

I’ll make a PR if you like the idea.

Something like this:

<some-chart>
  <template #tooltipTemplate let-model>
    {{model.customValue}} and {{some.outside.value}}
  </template>
</some-chart>

BaseChart would register @ContentChild('tooltipTemplate') tooltipTemplate: TemplateRef

And each chart would be responsible for plugging this.tooltipTemplate into the swui-template directive.

Note: I haven’t thought of a good way to override the area-tooltips. I won’t touch them in this PR.

Let me know what you think.

[marjan-georgiev]

This was implemented in https://github.com/swimlane/ngx-charts/pull/399.

[isaacplmann]

If both are defined, what if we ignore the ContentChild, since it comes later in the life cycle but also send a console.warn to the developer.