Add support for StreamGraph algorithm

### What problem does this feature solve?
ECharts currently supports the `themeRiver` algorithm for drawing area charts centered around the x-axis:


Another algorithm, StreamGraph, produces a similar but asymmetrical chart, that often provides a more visually appealing chart, which minimizes the slopes of the series for each point on the x-axis. See an example of the StreamGraph in Apache Superset using the NVD3 plugin:


There are two variations of this; the `wiggle` and `weighted_wiggle` algorithm (see the following technical paper for details: http://leebyron.com/streamgraph/stackedgraphs_byron_wattenberg.pdf ). E.g. matplotlib implements these https://www.python-graph-gallery.com/252-baseline-options-for-stacked-area-chart: (sym here equals `themeRiver`)


### What does the proposed API look like?
These algorithms could be added to the current `themeRiver` type as rendering options, or potentially introduce a new `flowGraph`/`streamGraph` type, and add `themeRiver`, `wiggle` and `weightedWiggle` as rendering options to it.

If this is something the project would interested in introducing, I would be happy to carry out the work for it.