Cone graph object: make the cone angle a configurable attribute

Use case:
In metrology, a cone plot can be used to represent the vectors for each point taken on a surface by a coordinate measuring machine or similar instrument
At moderate point densities and higher, the overlap between the bases of the cones makes the plot unreadable unless the cone size is dialled all the way down (in turn making the vectors' direction hard to distinguish).

Proposed solution:
Have the cone angle driven by a user-configurable attribute (with set limits) allowing for slimmer or fatter cones accordingly to the user’s needs.
Ideally a cone angle of 0 should be allowed to effectively transform the cone to a line of the same length, but this is a less critical point.

先找出錐體圖物件的 schema 和渲染實作，然後檢查現有的錐體大小屬性與相關測試。當使用者可以設定有界的錐體角度、在可行的情況下處理所要求的零角度行為，且視覺化在支援的邊界上仍然有效時，即表示完成。