You can filter your data by adding another transformation. The result will just be to show the data that is selected in the dropdown. If you need to show all data then will need to adjust that expression.
"transform": [
{"type": "filter", "expr": "priority == datum.priority"},
{
"type": "aggregate",
"ops": ["count"],
"fields": ["inc"],
"as": ["c_inc"],
"groupby": ["year", "month"]
}
]