[Feature] Tooltips on line series - also show on line segments

[ob-ARC]

What problem does this feature solve?

My use case is a line series with steps, i.e. it only consists of vertical and horizontal segments. Our users would like to see tooltips on the line segments as well, indicating the current value of y on the horizontal segments, and the value of x on the vertical segments. I would suggest that the data sent to the formatter are either interpolated (I assume that calculation is not performance-critical), or the data of the nearest data point is used (this was already mentioned in #15437).

Example:

https://echarts.apache.org/examples/en/editor.html?c=line-simple&version=5.5.0&code=PYBwLglsB2AEC8sDeAoWsAeBBDEDOAXMmurGAJ4gCmRA5AG4CGANgK5W0A0J6AthNCIAGbqVi9GGIgHYSAX1GxyOfEVRiK1OkzYdFfAcP3jJRAMzzFYYMGaQQanrDwALYAHciYAE7tLJPCpvCCpCWABtJ3UxWAgAEzpmASoARi4ndGhGXhpYWiToVPSYskpc_OTimLxyXgAjWzoAYwhvJuY9DOc3dwBlWobmL18qY3Qa-tteiAAvXJSRLrjGMEYicPCUzhSAXU4IgCZtvcPOA5PwszOLq9398IAWY_un8_uAVmuP54iANmO9l1rLZ7I4SugAPQQgBmwG8EjAYCCRGhrGgTUgMAAFCBGN5snh9pAmgBrKhgfZNFjMOqMUkASmQsG85NY3jgtCReDAtAA3OgFF10LD4Sskd46EhGHJkE05FUSq4PMN2GNSIwQNRoHEACrAABCwDi5BVVC6cicgrE0TE8USyQOCtIWRy9sKjrVmnKBU6ioGjTyLTaHSd4x6_UmQzIIzVE0G0zmRAWauWq3W4REdwiWzepwOADp3jczvmDkWXiXpOWIp8C9Xwv8UvmqztASVgXYIA5iODYCKEeKUWiMVBoDi8QSiRBSeTKdTaQymSywGyOVyefzYFbwVD-2LkXkpTKkHLQ6QlZ5o6qhbANVrdQajSar2aShaxO_YDsUHJeUA

What does the proposed API look like?

No additional API required.

[helgasoft]

if both axes can be converted to type 'category', then axisPointer could be used instead of tooltip - Demo

[ob-ARC]

if both axes can be converted to type 'category', then axisPointer could be used instead of tooltip

I know that, but it is not an option.

[ob-ARC]

If anybody is interested, the following example shows how a hint on the line (showing the closest data point, interpolation not possible as far as I can see) can be achieved by using mouse event handlers and the showTip/hideTip actions via dispatchAction:

https://jsfiddle.net/gjrn97pm/4/