problems with Contour Chart

Contour chart is [defined](https://github.com/apache/echarts-custom-series/tree/main/custom-series/contour) as _"connecting points of equal value (e.g., elevation or intensity) with contour lines"_.
But it looks like it's in fact a **density** (also called heatmap) chart. Library _d3_ is used to draw areas around adjacent points.
[This example](https://codepen.io/helgasoft/pen/bNVjEVy) shows several issues with current code:
* chart is not positioned correctly on X and Y axes, or axes are incorrectly drawn
* encode is defective for axes and tooltip
* dataZoom has problems with all _filterMode_ options
* tooltip shows the same value everywhere, it is the last value from data
* ```appendData``` does not work, maybe in general for all custom types (?)

Contours were discussed in [#13909](https://github.com/apache/echarts/issues/13909), and I said that _"contours are already feasible in ECharts with series type 'line' or 'custom-polygon'"_.
Maybe just rename this one from "contour" to "density chart" ?

PS: the heatmap chart with ```coordinateSystem:'geo'``` displays similar density effect - [example](https://codepen.io/helgasoft/pen/GRGqZeR).

Start in the custom-series/contour implementation and reproduce the linked CodePen examples. Investigate the reported axis positioning, encode, dataZoom, tooltip, and appendData behavior, then clarify whether the intended result is to fix contour semantics or rename the series as a density chart. Done means the chosen scope is documented and the affected behaviors work as intended.

Written by the indexing model from the issue text.