[feature] [topic] label enhance

Since there are many problems with the label, we should do some unified work to enhance the user experience of label. The issues referenced below are mainly related to three aspects question:

• label overlapping
• label show automatically with zoom
• multiple label, the label configuration is an array
• add click event for the label

For example, the following picture shows the label overlapping in themeRiver:

[pissang]

Thanks @deqingli

The enhacement of label is a long game. The final destination of label enhacement is all labels on the view are clear enough to represent the data.

Here are some of my thoughts about how to achieve it:

• First of all we should separate label creation from the data elements. That means all labels are all individual elements and can be handled easier.

• Provide label guide line(currently we call it labelLine) for all series.

• A general label layout algorithm which has following rules:

  • Labels won’t overlap. And may have a gap between labels.
  • Labels have local minimum distance to the original position after layouted.
  • Align labels based on given configuration.
  • Calculate a priority for all labels and hide the unimportant labels if the space is not enough for displaying all.

[pissang]

Hi @zevero, It has been implemented in https://github.com/apache/incubator-echarts/pull/12911