Time series plot based on unstructured mesh

[changliao1025]

For mapping, it is desired to use the geojson. But for time series analysis, it is also possible to develop based on the original netcdf output.

[changliao1025]

For structured mesh-based plot, the subset is often used because of the multiple-band datasets.
In the unstructured mesh-based plot, this is not needed because data is stored individually. But the subset keyword can be kept for consistency.