Hi Haikuo - I believe is the answer (from the Help Site) : "
Number of dimensions and measures
You can only use a single dimension type: point or area. You can create an expression in the dimension. If you add a measure from the master items to the map, the measure is added in the expression box under Data > Layers > <layer type> in the properties panel."
1: mouse the map, should show some tooltip, such as count(my_field)
2: use expression to create map color
I can do those in Qlik sense 3.0.
However, I can only use either 'by measure' or 'by expression' in the Colors->Custom part in Qlik sense 3.1, and then it is not possible to keep both the above '#1' and '#2' requirement at the same time.