Hi zongwei,
You can catch the field what you want. Use predefined tokens like $click.<field_name>$ to take'em.
If you want catch a value from field named foo , declare in your timechart drilldown tag. The same name field for you chart overlay in your case.
<drilldown>
<set token="clickedFooValueToken">$row.foo$</set>
</drilldown>
Token clickedFooValueToken will content the value from clicked field foo
You can find more info:
http://docs.splunk.com/Documentation/Splunk/7.2.0/Viz/ContextualDrilldown
http://docs.splunk.com/Documentation/Splunk/7.2.0/Viz/tokens
Greetings. jvardev.
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