[Feature] Add labels to continuous visualMap

[cgps-admin]

What problem does this feature solve?

Currently, only two labels can be displayed on either end of the visual map, it would be great if more labels can be added for the intermediate values:

What does the proposed API look like?

    visualMap: {
      type: "continuous",
      min: 0,
      max: 100,
      labels: true,
    },

[Ovilia]

Even with these intermediate labels, the mapping from color to value is not clear without hovering on the data to see the mapped position in the visualMap component. So I don't think the intermediate labels could help a lot.

[dberardo-com]

the issue is that for compatiblitz with echarts v2, as soon as one uses the text:[] attribute, no labels are shown. to me it would be very important, t expalin a title on top of the visual map to explain which dimension is taken into account. In my cas for example i am computing the piecewise visualmap on the basis of the Max value of the underlying series ... so it is key that the user knows this info